I've been a mostly 35mm SLR (now a no-film-but-still-35mm-style DSLR) photographer for about 45 years. Every photography book I read, or course I took; as well as my photo editor at the newspaper at which I would later work, and all my fellow photographers there and at the AP all recommended either a UV or Skylight filter to protect the lens. Back then I was shooting with a pair of CONTAX RTS bodies, and four Carl Zeiss lenses; and so I had really good stuff, and wanted to protect it.
I first tried a Skylight filter, but it removed a little too much blue for my tastes (warmed things up a tiny bit); and also reduced the amount of light that hit the film plane by every bit of a tenth of a stop... maybe a tad more.
So I went with a UV filter and, honestly, I could see little difference. Even the amount of light hitting the film plane seemed almost the same. Only if I used a loupe, and looked really hard, could I detect, in the images taken with the UV filter on the lens, such things as a virtually undetectable, otherwise, reduction in the details of things like the branches of trees in the far distance. Seriously, the UV filter didn't make enough of a difference to matter.
So I settled on a UV filter, and never looked back. Every lens I've had for nearly half a century has had one -- and a good one -- on it. Any other filters I might use get screwed atop it; and I'm careful not to accidentally remove the UV filter when I remove whatever other filter (always keep a pair of plastic filter wrenchs in your bag, so that you can grip one filter while you turn the other if you ever have to; of course, no filter should be on so tight that you ever need one, but I'm just sayin').
The UV filter, though, must, indeed, be a good one. Actually, better said: The filter must be really, really clean so that not a single spec of dust or a smear or fingerprint or anything can, in any way, redirect light. That's the key.
If the UV filter is a cheapo, then the statistical probability will be higher that its glass will have some kind of tiny imperfection in it; which imperfection basically achieves the same thing as dust or a smear or something. And so it's essential that oone starts with a high-quality UV filter; one that truly meets the definition of "optically clear."
Then, beyond that, it's just necessary to keep it very clean...
...starting with ensuring that not a single spec of dust is between the UV filter and the lens. If you can keep that part clean, then keeping the outside of the UV filter that's in contact with the world is a cinch.
But keeping the dust out from between the UV filter and the lends is actually trickier than you might think. You should see me put a new UV filter on a lens: first getting just the inside of the filter so clean you could operate on it, and then protecting it from dust while I do the same to the lens glass (being sure to get all dust out of the metal bezel around the lens glass, too); and then keeping dust out of there while I retrieve the filter and get it screwed on. Then I inspect and inspect and tap things to see if any dust dislodges and ends-up on the inside. Yikes! It's quite a little undertaking... and a sight to see, I'm told. It gives the phrase "anally retentive" a whole new meaning. [grin]
If you can get that part right, though, then, out in the field, you simply make sure that the UV filter is always clean, by normal means, which is easy. And the beauty of even having the UV filter to protect the lens is that if any of said normal means just happens to somehow scratch something, it's the UV filter, and not the lens, that gets scratched.
I'm sorry, but I wouldn't take any camera even out of the house without a UV filter protecting all lens fronts. Ever. No matter what.
Others, here, can disagree; but my pushing a half century of experience says that one should be using UV filters on all of one's lenses. Just make sure they're really good ones; and, of course, that they're clean, clean, clean.
Hope that helps.