Keeping old plates on the garage wall has become a part of the American culture. And vanities remind the American motorist about a part of his own history. Often, Europeans visitors collect American plates after traveling in North America, in San Franciso's Chinatown or Quebec city. Not as easy as for stamps.
Some collectors are just interested in codes, or share photos they took when traveling.
It's another way to look at one's environment. During the war in Bosnia, new plate designs have been a strong symbol of nationalism, as they are in some Russian republics. The easiest way is to get them in junkyards. Being a member of a collector association may help to show you're not going to use it for a stolen car, for instance. We'll pay you at least twice your postage charge.