which-key is more like autocomplete for keybinds. I consider it orthogonal to transient. It's meant for modals that show you what keybinds actually exist given some "prefix" you have just inputted. If I had to make an analogy, I'd say which-key is closer to company+counsel+ivy than transient.

I have used which-key with the three packages I just mentioned for as long as I have been using Emacs, and this is how I've always seen its role.