KenK writes:

Why? I doubt if they would do much, if any, good if the valve was leaking.
They get in the way when checking air pressure - if they weren't there I'd
likely check more often. All I can think of is to keep dirt out of valves?

Yep.

But the primary purpose they serve is in finding lost socks.
When you lose one sock, that sock goes to a special place.
The same place lost valve caps go.

Apologies to those who are not Ren and Stimpy fans.

--
Dan Espen