Familiarity for one; it was a great Reddit client and it's a good Lemmy one too. I certainly made the shift to Lemmy a lot more frictionless.
It also has a sensible layout, decent customisation, the dev is (or was, at least) responsive to the users and it was a solution that 'just worked'.
I paid for ad free ages ago, moving from RIF and I hadn't looked back.
It hasn't been updated in a while though, so I'm playing with Thunder and it's been a pretty nice option as well. Plus, you know, FOSS and available on Droidify/F-Droid. It's far and away the best 'native' Lemmy clientnive used to date.
Other than the ads, is there a compelling reason to?