Other endings (routes), allows the player to explore and read other main characters backstory and get a feel of their side of the plot.
Whilst true endings (route), usually would feature the main main main cover character of the game,
Yosuga no Sora, Sora route is essentially the true end.
G-Senjou, Haru Chapter/route
Hoshimemo, Hidden heroine true end route
Or, side endings can be used to illustrate the "What ifs"
And in special cases, in the Owaru to sekai birthday VN. It uses multiple routes to give hints and build on the true route/end, once all routes are cleared then the true end route will be unlocked.