Making and testing a backup should be part of any web administrator's regular routine. There are so many things that can go wrong. Not least of which is getting hacked or having malicious code implanted on your site.
Imagine if while checking your sites appearance in Google results you click on the link to your site and are faced with the following screenshot:
This is not good. It might be possible to remove the offending files but how do you know for sure that all is well. The best way is to wind the clock back to a happier time by wiping your site and using a backup.
As well as with nasty people messing with your site, it is not hard for an accident to occur. If you are using custom code that accesses the database then it only takes one wrong table to be deleted or even just altered in the wrong way and yours or your clients income is gone.