GUIs can have keyboard shortcuts too. I'm an artist and I work two-handed: right hand moves the stylus around the screen, left hand floats around the keyboard and changes tools, summons control panels, etc. Whenever I try a different program than the one I'm used to, and have to poke at icons with my right hand because I don't know its shortcuts, I feel like half my brain's idling.
The TUIs are a little funny, because often people can navigate and input faster than the program can render. So you vaguely see a screen change, a menu move or an overlay pop up, only to instantly disappear. The user doesn't need to know what's happening on screen, because they trust their fingers and the system to do the right thing.
I learned this while creating some autohotkey scripts: I could have the macro click before the button was shown on the screen.
This also created some problems where users would unintentionally hit their scroll wheel after clicking and change a 1 to a 7 on the next screen before it even appeared. That wasn’t good…
Yes, but the it's not due to slow rendering speed. GUIs render even slower.
Some frequently used functions have their own special single-key shortcuts as well, so instead of having to press C-P (to open the Clip menu and then select Paste) you can just press the ' key, saving the user a key-press every time they do that.
Doesn't work with reassignable keys, either. If I wanna reassign the shortcut for "pencil" to "d" because it's easier to hit with my left hand then that's my own business.