What is the point of Ctrl-S?

Long before there were computers, there were teleprinters (a.k.a. teletypewriters, a.k.a. teletypes). Think of them as roughly the same technology as a telegraph, but with some type of keyboard and some type of printer attached to them.

Because teletypes already existed when computers were first being built, and because computers at the time were room-sized, teletypes became a convenient user interface to the first computers – type in a command, hit the send button, wait for a while, and the output of the command is printed to a sheet of paper in front of you.

Software flow control originated around this era – if the printer couldn't print as fast as the teletype was receiving data, for instance, the teletype could send an XOFF flow control command (Ctrl+S) to the remote side saying "Stop transmitting for now", and then could send the XON flow control command (Ctrl+Q) to the remote side saying "I've caught up, please continue".

And this usage survives in Unix because modern terminal emulators are emulating physical terminals (like the vt100) which themselves were (in some ways) emulating teletypes.

It's a control character from the time of the VT100 terminals and similar. There was limited scrolling capability (if any) on those terminals to begin with the CTRL-S allowed you to freeze output to view what is currently on the screen without it being written off.

Most everything that you see in modern terminals was put in place to emulate those older terminal systems.

BTW, if you are listing a long output on your screen, chances are that you do not necessarily read as fast as the computer that spits out the output. Using CTRL-S/CTRL-Q on a cat (or active tail) is VERY useful to stop and resume output.