It's handy for keeping "tail" or "watch" commands or similar visible — the same reasons people use tmux, tiling window managers, so on.
The UX isn't perfect, but it's useful enough that I've stuck with iTerm despite the lower performance (and bugs — it's pretty buggy, and the main author rarely seems to address Gitlab issues).
iTerm has other nice features. It can run without a title bar (saves space), it does cmd-click-to-open-file, and it has a lot of customization options. I don't really use most of the features; the tiling aspect is the main feature I rely on.
My setup is to run MacVim on the left half of the monitor and then iTerm2 on the right half. iTerm is then split into generally three horizontal splits.
I love the tmux integration, I used tmux before anyway and it's honestly not that different if you used tmux's built in mouse support but focus follows mouse in terminal panes is a nice touch.
Displaying images inline is alright I guess but I don't actually use it that much.
There's a bunch of stuff listed on their features page that sound useful but I don't actually use (yet). Idk I suppose I haven't noticed any appreciable difference in speed.
As far as I know, in terminal you can't use cmd as meta key, which immediately kills it for me as an emacs user (furthermore in iterm2 you can set it up so that left cmd = meta, right cmd = cmd, which I find very useful).
screenshot: http://imgur.com/a/mFWYC