I think it emotion behind it,
if your not used to go out as a woman, or man, by dressing and looking as a atractive one would be great felling. it kinda give you something if your used to hide your body and not minding yourself that your can reflect yourself in a way who mach your gender and seam very atravtice. you feel sexy, and pretty in a way you might not feel otherwise, and you might be complimented and get happy. specially if you look sexy pervert guys will compliment you.
I feel thats the main reason why people do so,
beside that fashion and what looks good on you is something you learn,
everyone got to find there own style, and if your not used to dress up in women clothes you wont nessesary know how to do it so it looks the best. I dont think ftm got big risk for looking sluttish since guy clothes are more covering, but much women clothes can be..
I think after going out for a time you will getting used to it, you will notice more what kind of clothes your into, and you will know what looks good on you or what you can wear. of corse this is diffrent from people to people, but you will find your style.
I think many transpeople including myself, didnt look that great when we first came out,
trying to much, cutting the hair to short even if i could pass with long hair, or buying the clothe too tight so on.. so on.. but we all need to start somewhere..
I hope it make sence..