A sequel to a film that came out 13 years ago is going to be released in December. Most of us don't remember what happened on last week's episode of She-Hulk, and if characters start talking about unobtanium, it could cause a lot of confusion when the movie comes out. If you can guess which one is not in the movie, you have accomplished something.
The first movie in the series will be rereleased in cinemas. The trailer states that the movie will be showing in all formats, including IMAX, 4K, and 3D, for two weeks in September.
The last point is important. People can watch it at home to catch up before heading out to see the new one, but almost nobody will have the equipment to actually watch it in the way it was supposed to be seen, with a pair of 3D glasses strapped to their face. With the rerelease, you will be able to relive the experience of 2009, without having to think about world-building. You won't get the thrill of listening to "Boom Boom Pow" or "Poker Face" on the radio as you drive home from the theater to write an angry forum post.
I attended a screening of Batman Begins and The Dark Knight on the opening night of The Dark Knight Rises, and some theaters have done the same thing for other movies. Titanic is getting a theatrical rerelease next year, but it isn't because it's getting a sequel. Because of how complex the movie was and how long it had been since it was released, it feels like a necessity to see the prequels.