With restic I don't need some kind of special server daemon on the other end, I can point my backup destination to any mountable filesystem, or relatively dumb "bucket" stores like S3 or B2. I like having the sense of options and avoiding lock-in. [1]
As for GUIs in general... Well, like I said, I just finished several years of bad experiences with some proprietary ones, and I wanted to see and choose what was really going on.
At this point, I don't think I'd ever want a GUI beyond a basic status-reporting widget. It's not like I need to regularly micromanage the folder-set, especially when nobody else is going to tweak it by surprise.
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[1] The downside to the dumb-store is a ransomware scenario, where the malware is smart enough to go delete my old snapshots using the same connection/credentials. Enforcing retention policies on the server side necessarily needs a smarter server. B2 might actually have something useful there, but I haven't dug into it.