It's quite common for a left sidebar to leave white space at the bottom if it doesn't need to use it. I don't know if other sites fill the space because they need to or not... perhaps we've got used to always having something there on steemit.com. I wouldn't want to keep "My Subscriptions" open unless somebody's in that section (some users seem to be subscribed to every available community).

I'll attach a few examples that have a lot of "white" space which I think work well (Plex in particular)...