Do valve caps serve any actual purpose? Are there any consequences of just leaving them off, or are they simply decorative?
My WAG would be to keep debris out of the valve itself.
Their purpose is to keep crud out of the valve. You can run without them, but they are cheap so it is a good thing to have on your tires.
I’ve been told that snow can get jammed into the valve and could eventually deflate your tire. It’s a good idea to have those valve caps on. (Well at least here in “snow country” - Boston)
Waaaaaay back when I was a kid valve caps were these little, non-descript, black plastic things.
Not battery powered, glow-in-the-dark, strobe lights!
Dust or water between the face of the valve core and seat of the valve stem can cause leaks. (It’s especially a problem with water during freezing weather.)