It's true you can't reinstall system Apps, but I would say more than half of system Apps doesn't need to be installed in system, they also work as user Apps without problems, that is especially true for GApps except Framework and Store. So if you want some Apps back, you are only wasting some memory.
But in exchange you get the possibility to get rid of Apps you don't use but still in Autostart (like Samsung Apps, without third party App you can't even disable them) or install modded versions of them (f.e. Team Blacked Out Version of GApps).
This way you can get rid of a lot of unnecessary Alarms and Wakelocks you can't handle without root otherwise.