In my opinion there are a few reasons a girl would want a dick, for one, she is partly straight but mostly gay so she likes dicks but would rather have sex with a girl, so she wants her own dick to use for this purpose, which is probably one of the main reasons men have sex change operations. They could also just like dicks like snoodle said but don't wanna have a guy around all the time cause we can be obnoxious at times haha.