I stopped using iTerm2 in favor of the default Terminal because iTerm2 has worse performance and latency [1], and I wasn't getting anything out of its additional features. I noticed an improvement going back to Terminal.app. YMMV of course; but if we're asking why Hyper is necessary, I'd like to ask the same question of iTerm2.
Alacritty uses the gpu for acceleration, where iTerm uses it only sometimes. That might have changed in a recent release, but originally iTerm was limited in when it used the gpu.
I notices it a little bit when moving around quickly in NeoVim.
The only usecase that comes to my mind is text rendering when you want to navigate through some large log file, maybe even this some kind of syntax highlight.
Otherwise I'm quite curious too to why you may need a GPU rendering or something like this...
I wouldn't say that I need GPU to render my terminal, but if the alternative is to just let the GPU sit there, unused unloved, then I'll take the terminal that accelerates using the GPU.