Let's go in order, shall we?
Print Screen: used to print the (text) screen on a special contraption called a "printer" that today's younglings barely know about (I know, I know). That functionality, itself, has long been obsolete, but Micro$oft acted wisely (a rare move by MS) and repurposed it to capture the (graphical) screen to the clipboard. We have reached the point where the Print Screen key/keycap ought to be renamed to Screen Capture and everyone would be better off for it.
Of course, later on, the idiots at MS placed the interactive screen capture on Win+Shift+S instead of Shift-PrtSc, but that's another problem (perhaps they did it on purpose to undo the rare act of smartness from before).
SysRq (technically, Alt-PrtSc): originated in PC-DOS, to call up the lower levels of the operating system if anything went haywire on the application level (and boy oh boy did I put that to good use in the Windows 3.1 days!). Long obsolete, other than in Linux, which still uses it for basically the same functionality.
Scroll Lock: Undoubtedly, the most misunderstood and undervalued key in the entire keyboard. It was AND STILL IS used to switch from regular (cursor) scrolling to window scrolling, and it's exceedingly practical on applications with large virtual windows (spreadsheets like Excel being the canonical example).
Pause: pauses the output (shocking, I know). This used to be vital back in the day of scrollbackless text windows (a feature kids today take for granted, but for which we used to pay our hard-earned money — I'm not actually joking here!!!), but not so today. STILL quite useful, for those of us that are... um... Homo Sapiens Sapiens enough to actually NOT be afraid of using the command line.
Some games actually ACKNOWLEDGE the Pause key! And in a regular keyboard is easily reached for during a game, unlike, say, P.
Break (technically, Ctrl-Pause, NOT Alt-Pause): breaks the current execution or output. Again, vital in command line environments... and in quite some more programs that are graphical in nature. Older systems had a dedicated key for it, but IBM moved it to a secondary assignment under Pause, which was a pretty smart decision usability-wise.
Unix has long used, and keeps using, ^C for the same functionality, which has been a source of clashes between Unix-style and CUA-style assignments for cut-copy-paste (which used to be secondary functions to be overloaded elsewhere back in the day, but now should be the absolute front line candidates to get their own base keys).
And now, the elephant in the room... why is not Num Lock in this list? EACH of the supranav keys has more legitimate uses than this ACTUAL relic of a bygone era, and yet it is Num Lock that remains present in any keyboard with a numpad, and in the base layer, too. It is high time Num Lock disappears and frees up the key for better uses. And if not, do as the IBM Model M SSK did, and move it to Shift-Scroll Lock — still available but out of the way.