Because some things cannot be found with anti-virus programs. Sometimes when my pc gets an infection its found by hijackthis and not avg, and sometimes avg picks it up and not any other program can find anything. It's always good to have a set of tools to keep your pc clean.
Malwarebytes is a good one. Ccleaner is another good registry tool.
You have to remember, new malware comes out hourly...so there is always a chance something will slip through. You can either ahve a yourself a few tools to remove this stuff when (if)it happens or pay someone like me to get rid of it. I choose the latter