They're good without, so don't worry about the extra unnecessary expense.
The reason that some reptiles need UV, is that they use it to form Vitamin D3 which in turn allows them to absorb calcium from their food properly. Without UV, the process doesn't happen and they get MBD. These tend to be basking species like Beardies.
However, Corn Snakes have never evolved this dependency. They are able to absorb calcium efficiently from their food without sunlight/UV. Corns are most active at times of low or no sunlight and they don't bask.
Sometimes stores will recommend UV for Corns. This is usually because either a) They say that for all reptiles and view it as "covering their backs" because they don't have specific knowledge of individual species, or b) They want some more of your money!
