Wheel bearings

[Jack & Donna Smith]

Question not exactly Wanderlodge; but today, while in traffic, a 18
wheeler pulled alongside, Beautiful Alcoas. (I always notice all
pretty wheels since I scrapped my SplitRims)
But these wheels, on the rear of the trailer, had a 3/8" or so,
plastic tube coming out of the center of the bearing cap, and seemed to
attach to the valve stem, or somewhere near there......
Have been wondering what its function is all day.... Any Comment?
Jack Smith
1973FC31
SoCal..