Its brilliant for Marvel. They get to add arguably their biggest franchise character to their defining Crossover movies when they wouldnt have been able to.
Despite the garbage movies that keep getting made by SOny, Spiderman is a billion dollar franchise, and a brand that puts asses in seats.
I look at it like this:
before: Sony keeps making shitty Spiderman Movies and Marvel doesnt get to use their top star
Now: Sony will still make shitty spiderman movies but Marvel gets to oversee it to some extent, and they get their star.
And Sony gets the help it needs to make good spiderman films...maybe.
I agree with The Ancient, no idea how Hollywood execs did something that made sense like this.