So that's the nifty thing about Unix is that stuff like this works- when you say "locked up", I'm assuming you refer to logging in to a graphical environment, like Gnome, KDE, XFCE, etc. To an extent, this can even apply to some heavy server processes: just replace most of the references to graphical with application access.

Even lightweight graphical environments can take a decent amount of muscle to run, or else they lag. Plus even at a low level, they have to constantly redraw the cursor as you move it around the screen.

SSH and plain terminals (Ctrl-Alt-F#, what number is which varies by distro) take almost no resources to run: SSH/Getty (which are already running), a quick process call to the password system, then a shell like bash or zsh. A singular GUI application may take more standing RAM at idle than this entire stack. Also, if you're out of disk space, the graphical stack may not be able to alive

So when you're limited on resources, be it either by low spec system or a resource exhaustion issue, it takes almost no overhead to have an extra shell running. So it can squeeze into a tiny corner of what's leftover on your resource-starved computer.

Additionally, from a user experience perspective, if you press a key and it takes a beat to show up, it doesn't feel as bad as if it had taken the same beat for your cursor redraw to occur (which also burns extra CPU cycles you may not be able to spare)