Why would a girl want guys to dress as a girl?

there could be several explanations for it.

1. they may think its cute
2. they might think its funny
3. they might think you'd make a pretty girl(dont take that as an insult, most guys cant pull that off.)
4. it might turn them on. (i've had a few girlfriends like that)

or it may be something else entirely, you never really know a persons intentions unless they tell you.