I guess adb backup was before my time. I did use adb to transfer my apps when I last restored a twrp backup. In perhaps a similar manner to what that did, going by the name. But I did use adb root for that.

Otherwise, I use it to set a lot of otherwise inaccessible settings, like making the back gesture a lot thinner than intended because my touchscreen can handle it, or forcing 120Hz everywhere. I can also set my dpi there without anoying apps.

And ofc I use it to uninstall system apps I don't need.

After initial setup I do all of that in a root bash session in termux admittedly, but if I hadn't rooted my device I would still want to do most of that using an adb shell, as most of it doesn't require root (besides maybe the restoring backups part).

I also use shell environment to semi-automatically transfer media files for certain processes, though I'll probably move that over to syncrhing at some point.

The main remaining advantage is the ability to automate things on my phone from my pc, I don't see a lot of those as replacable unless my rom installs kde connect as a system app and they add an immense amount of functionality