Tire covers were considered pretty essential for track riders. Sub 200 gram tires don’t have much in the way of sidewalls.Tire covers do offer some protection when carry wheels and loading them for transport. Also, they do limit the amount of exposure to direct sunlight. Not a big deal some places, but worth considering in places like So.Cal and Colorado.
But it was not just track riders. Back in the days of the Campy Van( which should have its own thread ) Bill Woodall and his compatriots always covered the tires on the wheels that rode atop the van. They stayed cleaner, out of the sun, and were likely less bug splattered after on of their cross country trips. But Campy used the blue rubber Clement covers that were supposed to be some odd sized Italian tire. Never knew for sure, as the few times I got to talk to the guys in the van I had questions about more important things than their choice of tire covers.