The most common reason I've had to do this is that I've exhausted RAM and swap, and the system is disk thrashing for spare pages. In some setups, Linux does not

The most common reason I've had to do this is that I've exhausted RAM and swap, and the system is disk thrashing for spare pages. In some setups, Linux does not like to OOM a process when it really should. (That's adjustable, by the way.) Also, you don't need to go all the way through it. In my above example, MagicSysReq+RE or MagicSysReq+REI is enough usually. E and I are for sending SIGTERM and SIGKILL, so all the processes are now dead, and we have plenty of RAM. Sometimes it does take a bit of waiting: the system is disk thrashing. That said, you can then just resume, though you'll likely need to switch to a VT and restart X and other things. (But you get to keep your uptime.) There's also F, which invokes the OOM killer, though I often forget about it.