I use Twitter because the people I like are on there. They often get banned for dumb reasons, and I'd prefer they didn't, but the general network effect of Twitter is strong enough that people will just make new accounts and/or avoid posting stufff that trips the censors.
Plus, as some other comments mention, Gab does not in fact "not delete anything unless it is found illegal by a court", their TOS[0] says they can ban you for posting obscenity. Why would I want to go on a website full of white-christian-nationalists if I can't send them goatse?