Did you ever play Runescape? If you did, remember "Party Hats" or "Phats" or "Haloween Masks"? You had them because nobody else could have them.
These are collector items. People own them because they have some value, and nobody else can acquire them. Or very few people.
The most expensive collector's item in the game, Hector's Relax, you can actually use, but the point is most of which are for owning a very expensive item in which you can't just "find in the market". It's especially cool if it's named/made after your, like Master Siku's Toy Piano.