The advertised purpose of a Skylight or UV filter is to cut atmospheric haze by absorbing ultraviolet light.
Since ultraviolet light can also reduce overall sharpness, absorbing UV rays will help sharpen your photos. A skylight filter will also reduce atmospheric blue by slightly warming a scene. Practically, the effect on a typical photograph is subtle. But there’s another, more important reason to have a UV or Skylight filter on every lens you own: It protects the valuable front glass element of your lens.
If you were to bump your lens against something and it gets scratched, you’re stuck with it. If you clean the front element of your lens carelessly or with the wrong material, coating could come off, or the lens could get scratched when you wipe it. Remember that lens elements tend to be a bit softer than filter glass, and therefore more prone to scratching. If you’ve paid hundreds or thousands for the lens, and ruin the front element, you’re going to have to buy the whole thing again.
A UV or Skylight filter, if it’s scratched, costs significantly less to replace. In fact, a Tiffen 55mm skylight filter costs around $8, and while the price can go up to over $100 for special sizes and added coatings, a typical filter will cost you $25 or less. Spending $25 to protect a $300 lens should be a no-brainer.
One caveat: Get a multi-coated filter. These reflection-reducing coatings help reduce ghosting when photographing scenes with bright light sources. Uncoated filters (typically the least expensive) are likely to produce such ghosts because light bounces around between front element and filter, and while they cost less, they may be a false economy. Multi-Coated filters, such as the Hoya 55mm UV Multi Coated, should cost less than $25.