the term is "trackstand." it comes from the use of that technique during match sprints. youtube will have examples for you to watch.

i trackstand at lights because if i'm looking to make a quick gap, i have to be ready to ride for it. if i first have to get my feet in the pedals and my arse up on to the saddle, i won't be able to do it.

and because of vanity. if anybody says otherwise they're lying.

also, it's not just fixed gear riders. lots of roadies do it too. i do it on my road bike.