It seems the last few years I have seen more and more of them. Gives the truck a slightly different look. I originally had it powder coated for better durability but powder coat color was slightly off. I had a body shop paint it the second time and clear coat to get a better color match.
You can buy those top hats like what @Banker has for those steel can air cleaners from United Pacific or Horse & Buggy in California, Missouri their the trendy thing right now their not cheap but neither is Class
Never really cared for those "garbage can covers", on top. I like the original painted breathers with the small caps.
I think it is just another way today to personalize one’s rig and have a slightly different look than most modern trucks. I have always liked the look of the older trucks. Fortunately with a Peterbilt and some parts and money here and there it is possible to make it look older/different than it really is. A couple years ago at the Mid-America truck show I looked at a bunch of rigs with various air cleaners and caps and took a bunch of pictures to study. I am not sure yet whether I will do it, but I saw some with old school pinstripes that really looked good. My first Pete years ago was custom pinstriped and I may one day do that to this rig. Unfortunately the guy who did mine has passed and his son who took over the business is mostly retired. I follow a guy on YouTube that still does custom pinstripe.