Let's face it: the stock lenses on your DSLR aren't the very best. Their quality is decent for most jobs, but when you start enlarging your images to 100% resolution, you start seeing some slight artifacts and loss of detail that aren't always due to your image sensor. Megapixels aren't everything - and it's sad to see that even expensive cameras today come with a cheap 18-55 kit lens that can't really do much.
Now, some of you might have those old film SLRs at home. Or your parents might have them, gathering dust in the basement and with a bunch of different lenses included. They were quite popular in the '80s and '90s before being replaced by digital - and no one really cares much about them.