The departments were scored on a ratio of service plan dollars to total sales dollars sold...same with accessories. This was five or more years ago, but both were/are "cash cows", so I'm sure that hasn't changed.
In all honestly though, they really were/are worth it for some products...others, not so much though. Things like camcorders, digital cameras, cell phones, PDAs and big screen TVs were worth it. DVD players, small TVs (tubes), washer/dryer, printers, etc were not worth it IMO.
Typically the products that come with rechargable batteries, those that need maintenance, and high end products are worth covering.