What fan doesn't have a jersey of their favorite team?
It's way to express your loyalty and fanaticism in way that allows others to take notice. Fellow fans will walk by and give you a knowing nod - as if you both belong to an exclusive club. The opposition gives you a stare because deep down you know that they wish they could follow a team as cool as yours.
Then there are fans that have the ability to take the extra step of adding game-used apparel of your favorite player. Not that this is required of any fan since many game-used jerseys can sell for far more than what a typical fan can afford.