Why Would I Need An Extra Lens To Photograph My Artwork?

I don't know why you would need an extra lens, one lens should be good enough. I would say a 35mm or 50mm lens should be sufficient depending on what type of DSLR you are using. I would use prime lenses if you want to get the best detail out of your artwork. You can get a prime lens in either one of those focal lengths for a decent price.

Maybe a fixed lens 50 mm.

One of my friends has a DSL Canon and screwed one of fixed 50mm analoge lenses on it. He makes great razor sharp images with it.

The benefit of an analoge lens is that it is cheap, ebay, and a fixed lens makes sharp images with little distortion. Do check if it fits on your digital camera!

You should research DSLRs. I didn't see anyone say you needed an "extra" lens but for the price you are willing to spend, a lens wouldn't be included unless it's a used camera kit.