Together with Scroll Lock, Insert and Pause/Break, SysRq (short for System Request) is one of the atavisms in modern personal computers. Harking back to those optimistic days of innocent enthusiasm when each key had a Purpose, and often a glorious one, these keys now exist merely for backwards compatibility. Yup. Just like Windows.
SysRq - usurp console input from the active program and redirect to the OS shell
Pause - pause running program
Break - stop execution, return to OS shell
Ins - toggle between insert/overwrite text entry modes