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Before the release of Spider-Man: Far From Home, articles such as this one made the assumption that the film would be part of the Marvel Cinematic Universe's "Phase 4".

Even though Marvel hasn't confirming anything thus far, we actually know quite a bit about what movies are coming up in the MCU's Phase 4.

The only film that is officially confirmed by Marvel Studios is Spider-Man: Far From Home.

Some time later, studio president Kevin Feige said in an interview contradicted this assumption with the unexpected reveal that the film would be a "Phase 3" film instead:

Feige was speaking with Bilibili when he confirmed that Phase 3 of the MCU does not end with Avengers: Endgame, but will instead conclude in Spider-Man: Far From Home.

"It’s the end of the third phase. You’re the first person I’ve told that to," Feige revealed.

What is the significance of Spider-Man: Far From Home being a "Phase 3" film instead of a "Phase 4" film? It seems to me like the movie would have turned out the same regardless of how it was categorized, so I'm not understanding the significance of the move.

Spoilers for Endgame etc may follow. You have been warned.

The films wraps up the Infinity Saga by finishing the story of Tony Stark.

To quote Kevin Feige:

"We've been working for many years on Infinity War and Endgame and Spider-Man: Far From Home. Those were the films that were culminating the entire Infinity Saga of the MCU. And as we were working on Endgame we realized that the true end of the entire Infinity Saga, the final film of Phase 3, had to be Spider-Man: Far From Home, because - spoiler - we lose Tony Stark at the end of Endgame."
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"The relationship between Peter Parker and Tony Stark is so special over the five films that Tom Holland has portrayed Spider-Man in the MCU, that we needed to see where his journey went, and see how does Spider-Man step out of the shadow of his mentor, Tony Stark, and become the true hero that he was always meant to be. And it's for that reason that Endgame and Spider-Man: Far From Home are essentially two pieces of the same story, and it's not over yet until Spider-Man: Far From Home."
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Far From Home gives us the story of how Peter Parker is dealing with the loss of his mentor and father figure, Tony Stark. This is tied pretty closely with Endgame, since, well, Tony dies at the end of that movie, and we've seen in Engame, and previously in Infinity War, Homecoming, Civil War, etc, that Peter and Tony care deeply about each other.

Far From Home had to wrap up Phase 3 by finishing the story of Tony Stark. Tony was a major player for all three of the first phases, but now, with Phase 3 coming to an end, Tony's story is coming to an end with it. So Far From Home had to end Tony's story, and, because it's connected to Tony, had to be a part of Phase 3.

If you take a look at Dramatic Structure you can easily fit the MCU at it's current point into this particular structure.

All of the movies pre-Infinity War make up the exposition, ie the background information for the main conflict. Infinity War and End Game make up the rising action, climax, and falling action. These are where the main events of the story happen. And then finally Spider-Man: Far From Home makes up the Dénouement, the release of tension and the eventual return to normalcy.

Essentially, FFH serves as a book end for the Infinity Saga and helps clear the slate for what's to come.