I usually use the gallery for houses, because I can't build, and I have found builds where a family of 8 can move in without cheats.
I also like to use the gallery to download sims, I tend to make fandom sims, like Harry Potter, but I also like to go online and download sims that people have already made for fandoms. The reason I do this is due to people making some of the same characters unique with how they picture them, or represent them.
I also found really cute sims pets that resulted in really cute pets from breeding, which I posted on the gallery.
When I have time, I plan to create more fandom Sims, I tend to make canon, semi-canon, alternate, and other versions for the same characters. I also love making them live in the same world. I currently have a bunch of Frozen versions in my gallery now. One is actually a gender bent version of the Elsa and Anna as teens.
So, in conclusion, I use the gallery to download a bunch of things for my game, and use it to post fandoms.