Why do some people cosplay?

They do it because they enjoy it. It provides some entertainment value and for groups of friends that cosplay together, it’s a social thing. There is also a lot of personal pride involved for the people that make their own costumes.

^
That.
Especially among video game and anime fans, cosplay is a hobby and a passion. Seems to be more of a Japan thing, but there are plenty of conventions here where people do that. I knew this guy a while back who spent months making himself Link costumes (Legend of Zelda) just to wear them once for three hours at some convention, but he was really into it. Some of it can be pretty amazing, too.

What is it?

Costume play. Halloween outfits not on Halloween.

Adam Savage from Mythbusters is a cosplayer, too.