They’re useful for printed media

“Find more info at bitly/event” instead of “find more info at facebook.com/unnecessarilylongurlthatnobodyisgoingtotype”

Or for a qr code where it needs to be small and somehow your URL is too long

But yes, relying on a third party company for something that needs to last a long time isn’t really smart. There are many books that have online content now inaccessible because they used a link shortener that’s dead or that doesn’t let you update the redirection without paying a ransom (need to pay $120/year to bitly if you need to change the redirection)