A caster usually gets the ability to cast a Power Word Kill spell only at level 17. The HP of players and monsters at level 17 are significantly higher than 100. The player can’t find out the exact number of HP of a monster, and he has only one attempt.

What benefits does this spell have to make it worth using?

When 0 HP is not the same as dead

One of the biggest advantages to Power Word: Kill is that the target is dead, not just at 0 HP.

I can think of several situations where an enemy having 0 HP is not the same as the enemy being dead.

(Side note: This is in large part why it could be argued that this spell is not intended for PCs, as PCs don't die at 0 HP. However, the question is asking from a player's perspective, so this answer focuses on that aspect.)

Enemy healers

While it depends on your DM, not all enemies die when they reach 0 HP. Optionally, the DM may choose to have death saving throws for an enemy; or worse, there may be an enemy healer in the battle too.

Think of all the times you've seen someone cast Healing Word, or put a Goodberry or Healing Potion into an unconscious ally's mouth. Such a tactic is very powerful at getting another PC up and running immediately.

A DM is free to use that same strategy by including powerful healers as part of an encounter. Knock the big enemy down to 0 HP? Good job, until it's the healer's turn and brings them back up to positive again.

Vampires

Vampires have the Misty Escape trait:

Misty Escape. When it drops to 0 hit points outside its resting place,
the vampire transforms into a cloud of mist (as in the Shapechanger
trait) instead of falling unconscious, provided that it isn't in
sunlight or running water. If it can't transform, it is destroyed.

When fighting a vampire, Power Word: Kill can be the difference between a challenging encounter and a recurring villain.

Polymorph/Wild Shape

Theik's answer covers this well.

In short, a Polymorphed or Wild Shaped creature reverts to their original form when they reach 0 HP. With Power Word: Kill, they just straight up die (and their dead body reverts).

It is irresistible

Many spells have the ability to save to reduce or avoid damage. Some creatures have Legendary Resistances, which allows it to save a limited number of times per day, even if they fail their saving throw.

No such save exists for Power Word: Kill. The only condition is that they have 100 HP or fewer, and then they die.

It has only a Verbal component

There are no material or somatic components to this spell, which means you can do it even if you are missing your components pouch or have your hands occupied with other things.

Not all tables pay attention to such restrictions, but if your DM enforces having a free hand (or something that fulfills the requirement) in order to use somatic and/or material components, this makes this spell potentially very useful.

Situational, but simple

In short, the spell is not a generic "I win" button, but rather a situational tool that can be used in a variety of circumstances.

It is not going to be the best choice all the time, but it can easily be the best choice some of the time.

It is far from useless

Okay, you don't know how much HP an enemy has, but that doesn't matter. If you guessed right, the target dies. That in itself is already pretty darn good. Resistances don't matter, there is no saving throw, if you guessed right, they're dead. This in itself is something no other spells can do.

Kill shapeshifted targets

But there's a fairly easy way to combine this with a second spell to get a guaranteed instant kill, even if it is a bit cheesy. If you polymorph your enemy into something which you are certain of has less than 100 HP, you can then proceed to kill them instantly with Power Word: Kill.

They will then revert back to their original form because polymorph ends when the target dies, but it's still dead, so for the cost of two spells, you've killed a target. Polymorph is a fairly low level spell, so you can just keep trying until you succeed, if they don't break free before your next turn, they're dead. This same trick also works wonders against Moon Druid opponents, who can keep shapeshifting to regenerate their HP.

Determine their current HP

Alternatively, get a Battle Master friend!
At level 7, they get the ability 'Know Your Enemy', which will allow you to study a target for one minute to determine some of their metagame properties.

The DM tells you if the creature is your equal, superior, or inferior in regard to two of the following characteristics of your choice:

Current hit points

Your fighter friend will surely know their own current hit points, so if they're sitting at 100 or 101 HP, they'll be able to determine for you if the target will be vulnerable to power word kill.

This doesn't work in-combat, but it does allow you to determine if you can swoop in and instantly kill the target, as long as your fighter friend is capable of studying the target for a minute.

When the target absolutely, positively, unequivocally must die NOW.

Theik's answer covers some incredibly clever and useful ways to get more out of this, so I'm not going to go that route.

Instead, I'm going to address the situational factor of this spell. From my experience in games, when you are at a level high enough to access this spell and an enemy has less than 100hp, the odds are that the party will kill it in a round.

Because of that, I generally found this spell underwhelming. To commit such a costly resource for something that will go down anyway with much less cost in a round generally never made sense.

Generally.

But there are situations where you and your party are really hurt. Like may not survive another round hurt. If you know that the target has taken quite a bit of damage over the encounter and think it's a good opportunity (n.b. there is still risk because you don't actually know), then you can unleash this spell. If it can end the encounter immediately, that may be the difference between a successful win for the group vs a TPK.

Not All Spells are Meant for PCs

This spell is probably inferior overall to Wish, Shapechange, True Polymorph, Meteor Swarm, etc. and as a PC I don't think that there is any situation that has actually come up in play where I would have been inclined to prepare it over the alternatives, even if I were playing an Enchanter who could twin it for free. However there is something that you need to remember as a player that it's easy to forget.

Not all spells are meant for you.

For example, Liches have access to Power Word: Kill. For all intents and purposes, you're probably not fighting a solo Lich at level 20 - you may be fighting one at Level 13-15. Well, a Level 13 Wizard doesn't necessarily have 100 HP. As a result, PW: Kill is a scary spell when used against a PC.

Were the Lich to use a different spell, however, (say, Meteor Swarm), that may result in a party wipe if the Lich is played to its intelligence. So that's the benefit - it's a scary spell that is going to have a big impact, but that's not necessarily going to result in a party wipe.

Niche Uses

Yes, the spell has some niche uses. 0 HP is not necessarily "dead" for all monsters (though in practice it is 99%+ of the time), which includes L20 Moon Druids and Vampires. This is a very niche scope - remember, we're talking about level 9 spells here, and you're almost always going to have better options.

Most relevant monsters at L20 have far more than 100 HP, making the spell kind of irrelevant much of the time.

If I knew for certain that I would be dueling a Moon Druid 20 (or taking on multiple Demiliches as an Enchanter or some other situation where the spell shines), I might consider picking up PW: Kill.

How often is this situation likely to crop up? Probably never, so it's kind of a moot point.