Bobby.. I worked for a cable company for several years, and we were constantly getting checks from people who switched envelopes. For a $39 cable bill, they'd send in a $700 rent payment. Adding your account# on the check helps to make sure that things get credited to the correct accounts. Also, I have two cell phone bills, and I have to be careful that they don't credit the wrong check to the wrong account.
To quote from the movie Starman (1984):
Red means Stop, Green means Go, and Yellow means go very fast....