Hubcap has the following functions: 1. Waterproof and dustproof, and then anti-corrosion. As can be seen from the picture, it blocks the bolts that fix the wheel hub. As the questioner said, on dusty roads, Hubcap can largely isolate the bolts from dust and water - the friction generated by the accumulation of the former will destroy the bolts. Anti-rust layer, which further aggravates rust. It is unlikely that the wheel hub will fall off during the race, but during maintenance, corroded bolts or bolts that have accumulated dust will require extra effort to unscrew. And because the Hubcap is snap-on and not fixed by other bolts (that would be meaningless), there is no corrosion problem. Early Hubcaps also had the function of protecting wheel bearings. But in today's cars, the Grease Cap and Hubcap that protect the bearings are separate. The function of Hubcap is actually more obvious in civilian vehicles, and its uses are quite extensive, because after all, the frequency of removing wheel hubs from civilian vehicles is much lower, and it is particularly important to avoid the accumulation of dust and moisture:

2. Appearance effect. It's even less obvious on racing cars, but the hubcaps of civilian cars often have the manufacturer's trademark, so there's no harm in publicity, and the shielding bolts also play a nice role in aesthetics.

3. Reduce air resistance. I want to talk more about this. At the beginning, I thought that the reduction in air resistance was more the result of Hubcap than the reason for using it. In other words, I felt that its impact on air resistance was negligible. Some drivers in the Unlim 500 drag race will seal the gaps in the sheet metal of the car's front face (or even the car logo) with tape before the race to reduce air resistance, like this: