I've always considered Maximum Cards an aspect of cover collecting. From the US, this is one of my favorites, though not as colorful as the newer material, but it's nearly 50 years old now (so these type of cards have been around for awhile):

The "problem" with these sort of side collections is (a) proper storage; and (b) what to do with them when you acquire an abundance or duplicates (as most cover collectors don't have much interest in them on the secondary market.)