Here are things I’m currently using root for on LineageOS:
System-wide adblocking (hosts file with Adaway). Yes, there’s DNS-based adblocking, but sometimes that can be slow or not play nice with public wifi networks.
Battery management. I use AccA to limit my battery capacity and charge rate to keep it from wearing out. I might not care as much if the battery was easy to replace.
File management, as there’s an increasing amount of the filesystem blocked off without it.