Via cron scripts you can fire up any executable script on linux at any time you wish. For example you can send out a newletter every day around midnight, clean out uploaded files every hour, checking for incoming mail every minute, etc, etc.
To find out more about cron checkout the man page, which explains it all.
What you need to be aware of when running a php script from cron, is that 1) cron doesn't have the same PATH you do, so mostly just calling
php won't work, it needs to be
/bin/php or something like that (type
which php to find out where php is located on your system). And 2) the current directory when running the the php file doesn't necessarily have to be the directory the php file is in.
The best solution for both 1 and 2 is to use absolute paths for everything (or use DIR (or dirname(FILE) for pre-5.3 php versions))