Something has to happen for Peter or Tony to say, "time to upgrade". His current costume alone already had an advanced combat mode and tons of other upgrades.
I'm betting Spidey gets hurt and Tony calls in the suit to help him.
Nah, my guess is that Peter sees the big Wheel in the Sky and immediately resolves himself to go in and do his part to help protect New York City as Spider-Man, but Tony goes all Protective Parental Figure on him and says he's only allowed to join in if he takes the upgraded suit for this.
The big thing that I'm noticing about the new suit is that it seems to be way more "armor-y" than Peter's regular suit. It's probably less about Peter having all the bells and whistles he got in Homecoming and more just that Tony wants to make sure Peter is wearing more protective gear for this.
I mean, just look at Peter's track record as Spidey so far in the MCU...
1. Fighting several Avengers in a brawl where no one had any intention of killing someone and Spidey most notably fights a Captain America that's going easy on him.
2. Two different Shockers
3. Vulture
And now he's trying to take
Thanos on one-on-one.
Meanwhile, Tony already saw Rhodey get crippled by accidental friendly fire from Vision in Civil War and has been established as being overly protective of Peter in Homecoming with his attempts to try and get Peter to stay on low and let other people deal with Vulture.
My other guess is that Peter and Tony's beat-ups from Thanos in the trailer are probably in direct relation to one another. Thanos beats up one of them and the other tries to step in to save them, only to also get beat into a fine pulp.