Themken: True that when not knowing how to (like me and the Sys Req) we have no use for them.
Let me take this moment to remind you that the keyboard layout is different in different countries. I use several languages my keyboard was not really made for so need to get some special letters using combinations.
The Windows/super key is used for a lot of shortcuts in Windows and Linux, and presumably in OSX as well.
But in windows in particular, it's problematic.
Sysreq is the print screen key. Tapping it while in windows or linux puts a screenshot into your keyboard. Some chat programs (like skype) will allow you to immediately control+v this. Often times screenshot programs automatically key this. I think you tie this button with something else to use some sort of built in screenshot method in windows 10. In games, usually f12 or this key is used to make screenshots.
Right now, my method of getting some of those special letters requires changing layouts with alt+`, which can be a pain with too many keyboards. Right now i've settled for just english and japanese, and i google whenever i need a fancy letter.