I personally think Jack Smith is after three things with this:

1. He obviously wants any DMs.

2. It's possible Twitter is storing incomplete and draft tweets, and that Smith wants this data (which would explain twitter fighting this so hard).

3. Smith wants to go after whoever else was posting on Trump's behalf. I think the main purpose here could be to identify anyone involved in posts that were Q proofs, since it's possible Trump was not the one who posted all of these... but perhaps used Scavino, etc. Being able to harrass these people with his probe might be the cherry on top, with the other data around those q proofs being the main dish (since this may help them dox the people behind Q). This would explain the gag order as well, since they wouldn't want Q etc., to know they were doing it.