For me I primarily add things to the Library through Plex Sync. But with that still in Beta mode and having hiccups it becomes invaluable to be able to easily access an add to library button like was always present.
Often too there are full discs I purchase (BluRay and DVDs) that can’t be played back through Plex because of how Plex functions, but I do still own so a manual add is a necessary need.
And similar to zax2000 I use the visual green highlighted icon of it being in my Library to ensure that something hasn’t gone awry in collecting the item. Which could either be a mismatch in Plex, or a failure of some other aspect, e.g. OTA DVR recording.