I used to believe the filter was a protection device too, and had one on every lens I own. I even have one of these stories:
As you can clearly see, that's my 100-400mm f/4.5-5.6 L IS, in a shot taken just after flying off the front seat at a high rate of speed when I had to lock up the brakes one day. I had a heck of a time getting that filter off afterwards. I ended up taking it to a mechanic and asking if he had a big rubber oil filter wrench that I could try on it. With him holding tightly to the lens, and me cranking on the wrench, we finally got it off. He said he was sure with all the effort we put into it that we would damage the lens in the process but, thankfully, it survived. I promptly bought another filter and put it on to get "protected" again.
Then one day more than a year later, I was cleaning my sensor. To inspect the job I'd done, I put on a lens, stopped it down as far as it would go, pointed it at a white wall, and took a shot. Then I loaded the shot up in Lightroom to pixel peep it, looking for any remaining dust spots. I'd done a good job and none remained, but something else bothered me: faint rainbow colored rings. I looked at the white wall. I looked at the rainbow rings. Huh? I looked at the lens. I looked again at the rainbow rings. Huh?
Then I thought I should just try it without the filter, just to see what happens. I honestly didn't expect removing the filter to change anything, but I was trying to figure it out, and I wanted to be thorough. Filter off, I took another shot and examined it. No rainbows, just white. Hmmm... Filter back on, take another shot, rainbows are back. And these were expensive filters too. I justified the expense of better quality filters with the idea that I didn't want to put a cheap window in front of my expensive glass.
That was the end of my filter use for "protection". The only filters that go on one of my lenses any more are for a specific task, and that's either a polarizer or a neutral density filter.
YMMV
