If you use the GNU/Linux operating system intensely as development environment or as a server you'll probably find yourself in need of running a determinate command or script every X seconds forever.

This could be useful for example to check that a service you run on your server is working correctly (e.g. your Apache httpd server), to clear some cache your system is using or to periodically check for software updates. Personally I needed this to periodically update the DNS entry of my home server with its current IP (my ISP gives dynamic IP, not static ones).