For me it was just the convenience of being able to have most of my cloud things in one place (like calendar, reminders, contacts, notes, etc). I suspect it's probably not the best solution for each of those individually, and I'm definitely curious what other options are out there, but it's quite a convenient drop-in replacement for a lot of Google's cloud offerings for people who are looking for alternatives. Also I should note I don't actually ever use the nextcloud UI itself, I don't disagree it can feel a bit clunky sometimes. I use 3rd party clients and use my personal cloud to sync everything via webDAV and cardDAV.