I use UV filters on nearly every lens I have, mostly for protection.
|Mohs scale [Wikipedia]|
When you accidentally drop your camera in mud (= water + sand, as I did once), cleaning a filter is much easier than the front element as you can take the filter off and clean it under water so that you don’t have to wipe all that mud away and risking scratching your lovely front element. Plus, the outer part of the front element is serrated and it’s not so easy to remove the mud stuck in it (I spent quite some time to have it fully cleaned, and there's still some left). Plus, some lenses require one to be fully sealed (as my 16-35mm).
Now, there are filters and expensive filters. Without going through the whole debate about the impact on image quality, I just want to say that I have now a B+W "F-Pro" filter on my 70-200 and a Sigma UV (slim) on my 16-35. Well, construction-wise, the Sigma is… not so good. The glass part moves and bends when I put some pressure on it and I can see some debris between the metallic mount and the glass (yes I know that slim constructions are ment to be small so it’s normal that they aren’t as resistant as normal ones but you have to be aware of that). So, if you’re using your lenses in sandy/dusty/hard environments, don’t try to save 20€ and spend the cash on a durable filter even if itsn't slim. There's only 1mm difference between a regular construction and the slim profile. Vignetting is easily removable in PP. So don’t buy a 6th bag. Or another stupid accessory you don’t need. Or a new camera you won’t reall use. Or anything else that follows the same idea, and get some german über-hardened steel filters (just like the B+W).
|B+W 77mm UV filter [Adorama]|