The more professional looking a shop is, the more likely people are to have confidence in your ability to produce something worth buying, that's my take on it.
I've just updated my banner because I thought it was a bit drab. I'm pleased with the results, it's added a new sparkle to my shop front. If you make the effort to get one done too, I guarantee you'll be glad you did!
There are people on Etsy who make them, I think, but they're easy enough to do in Photoshop etc. Or if you choose a free one, simply add it as Missy Corey describes above. :)
