Cron Jobs run a script (say a .php file) without you having to run it yourself.

The only instance I'm using one in is updating a database with a .php script. This script grabs a daily-generated text file, splits it into the values to put in the database, and then updates it accordingly.

But other ideas for using cron could be:
- Daily forum pruning
- Removing 'dead' users once per week
Most management/admin things that are boring to you really but need to be done regulary are good options for cron.

I'd be careful about using cron to submit sites to search engines as they may not like it ^^

As for posting to multiple websites this couldn't really be done in cron. Cron is a repetitive automation bot that acts as a user - but does things over and over again.