It's probably less effort to get this t-shirt than it is to get achievement badges or hats or whatever for many video games, and this nets you an actual physical achievement badge.
If it does motivate people and they like them, then fine: I guess it's just different strokes for different folks.
It pretty much boils down to this. Not only are vendor t-shirts actually a physical item but they are also often uniquely designed; many vendors only provide certain designs for certain events. So they really become a badge of honor and can gain in emotional value: "been there, got the t-shirt..."