Inhibition from org.freedesktop.PowerManagement.Inhibit and org.freedesktop.ScreenSaver are handled pretty much the same way with preventing both sleep/suspend and screen power saving/screen locking. 
But that's absolutely illogical as far as powerdevil supports differentiation between these two types of wakelocks (e.g. on laptop lid close when external monitor is present).
So intentions for developers using these inhibitions are absolutely clear, just take a look at chromium (chromium/services/device/wake_lock/power_save_blocker/power_save_blocker_x11.cc:282) or ktorrent (ktorrent/ktorrent/pref/generalpref.ui, ktorrent/ktorrent/core.cpp:972) sources. 

This behaviour makes system keep screen on when no one app really needs it, like chromium/vavaldi/other forks playing audio (not video) or ktorrent downloading something.

It appears at least in KDE Neon, Fedora 26 and OpenSUSE Tumbleweed with 5.10 up to latest Plasma versions.