I'll echo what Anbesol said - their only purpose is to protect the front element of your lens. Camera dealers push them hard because they have a high profit margin compared to cameras and lenses. But your photos won't benefit from them in any way. It's not bad to get one unless you buy a cheap one. You don't want to spend $500 on a lens and then spoil it by putting a crappy, $20 piece of glass in front of it.