One reason is to move from being the product to being a user, so that your attention is not sold to the highest bidder and your behaviour manipulated in such a way that it produces the highest value. Of course, paying with your attention is a valid way of paying for things as long as the implications are clear.
IMO, federation is a better model of this. With Mastodon, sure somebody is still paying server costs, but there is no unavoidable fee for the software/service and it has the same censorship-resistent properties.
(I'd like to see signed messages in Mastodon but that's not a problem inherent to a federated model)
Censorship resistance. Twitter could delete your account (or some of your tweets) if they wanted to. They could also prevent your tweets appearing in other people's feeds. And they have done this to some controversial figures.
Basically a Dapp is a more neutral platform that costs money. Probably not worth the cost for the average person though.
If the price is meaningless amount I would actually be more comfortable paying than being a "product" that is being sold.
Especially if there is no friction in paying and apps would subtract little amounts of cryptocurrency and I could review from time to time if all is in check
edit: spelling mistakes