Guess this is mostly supposed to appeal to rich people and maybe businesses, not sure who else would want to pay a subscription to Twitter.

I have wondered about this, too. According to this, the service targets "power users". Not much help, there.

One aspect of Twitter Blue is apparently that it gives access to a bunch of advertising-free content. I guess the $2.99 is cheaper than it would be to buy separate subscriptions to all of the covered web sites, so it might appeal to high-volume news consumers, whoever that may be.