Are you responsible for the site?
It's embarrassing to hear from a customer that your site is down.
All of your various service providers will send alerts and notices, but what really matters is if your site is up.
You could probably build a script to check the site every 5 minutes and email you if it's down - but we have it all debugged already and have pretty charts and reports.
In fact, you probably have lots of more important stuff to do than solving an already-solved problem.
UPZO - for the price of a beer we'll cover this for you.