Useful feature of the LinuxKernel. Well useful when you control access to the hardware.
If you have a MagicSysKey enabled in the kernel, and have the correct proc entry set then you can get your machine to do a few emergency procedures in times of dire need. For instance, you can cause the machine to reboot without having to run a program. This is important if you have for instance used up all your memory and you can't run another program.