In the early days of motoring wheels were often made of wood. As performance increased they started to use steel wheels. Bicycle style wheels were popular for over 60 years. The optional variation on these were pressed steel wheels. probably for no other reason than cost, Makers created ugly old pressed steel wheels and then hid there ugliness under some kind of trim.
Hubcaps were the old variation of wheel trims and they were basically a plate shaped cover that was banged onto the wheel to hide the wheel nuts.
With the advent and popularity of Alloy wheels it was not a major surprise when wheel trims arrived that pretend to look like Alloy wheels. Most wheel covers today come in a sort of mock plastic pseudo Aluminium colour and mimic the style of Alloy wheels.
Wheel trims have no function for the user other than to disguise the ugliness of the wheels, which manufactures often don't bother to do anything with, other than paint them a dull black.
Wheel trims are though often expensive and probably an invaluable source of cash flow for some firms, which probably explains why many feel the need to eject themselves from the wheels with alarming regularity.
It would be probably be a hobby requiring a couple of acres worth of field, if somebody was to start collecting just one of each type ejected involuntarily from cars in normal road usage.