I believe his main goal is turning Twitter into something of an "everything" site where he's looking to combine the likes of Facebook, Instagram, TikTok, YouTube, Discord, and Venmo/Paypal into a single service. I don't know if it was his goal when he bought it or if it has any chance in hell of working, but that's something he at least said (while probably drunk) fairly recently.
There’s a woman over on Mastodon making the argument that he’s trying to destroy it so it can’t be used to organize left-leaning political action (fundraising, activism, issue messaging, etc) any longer. She makes a compelling argument.
I saw this article posted in another thread and it seems to explain a lot. Not sure how close it is to reality, but it looks like nostalgia is at least a partial reason for this change.