Your camera already has an internal UV & IR filter just in front of the sensor. The original purpose of these filters was to block light which wasn't in the "visible" spectrum for humans (but film could be sensitive to it anyway) BECAUSE the light at different wavelengths focuses at different distances. So literally your images would look very slightly out of focus in the image due to light your eye can't even see in real life.
But now that the camera has a built-in filter, there's no need to put the filter in front of the lens. Some people want it for protection though there are many arguments that claim that's not such a good idea. I can think of a few exceptions, but most of the time the lens filter will degrade the image quality rather than help it and won't really offer protection in most circumstances.
The biggest problem, however, is "ghosting". Glass surfaces are reflective. They all act at least a little bit like a mirror. The reason you can see a reflection of yourself in "clear" glass is because that glass technically reflects some of the light trying to pass through (you never get 100% transmission). If you have a filter, the light that passes through can reflect off the front of your lens, onto the flat filter surface, and reflect BACK into the lens again... creating a "ghost" reflection in your image that degrades image quality.
The best defense against this problem is to not use the filter at all. The 2nd best defense against this is to decide that if you really must use a filter, buy a higher quality filter with good anti-reflective properties (that will substantially reduce the reflections... but it won't completely eliminate it.)
Here's an example:
The filter on the left is an expensive filter and lacks anti-reflective coatings. The filter on the right is a high-end UV filter that includes the anti-reflective coatings.
Notice the black card-stock below the filters. On the left you can see the glass is giving off a shine that reducing contrast to the black card-stock below. On the right you can see the card-stock so clearly that it almost looks as if I've just placed the black filter "ring" on the paper and there isn't any glass in it (there is.. it's just hard to see because the glass doesn't provide much of a reflection). That's the anti-reflective property you want in a good quality filter.
While I do own these filters, I don't leave them on my lenses. I'll use a filter only if and when I think the situation merits it.