Valve stem caps on tire/wheel valves are hugely important, which is why they’re there in the first place although too many riders ignore the dangers. We see numerous motorcycles through the workshop that come in without one or even with both valve caps missing. They’re there to stop the muck and crud getting in and deflating the tire, but also to minimize air escaping if the valve leaks and to contain it as much as possible.
If you look at a brand-new valve, it’ll come with either a plastic cap or a lightweight aluminum cap that’s plated to stop corrosion. Some people seem to like going out and buying fancy valve caps for whatever reason, and wonder why their tire is losing pressure or the valve is bending. Just because your friend down the pub reckons it’s cool to rock skull head or dice caps, it doesn’t make it acceptable.
They are often cheap, porous metals that do more harm than good. You don’t really want that, do you?