Well... I can try to put it into words. It's like a roleplaying game, but you never have to pester the GM, the gameworld is shockingly realistic and vivid, and everybody stays in character. It's like bungee jumping, because it's really terrifying/thrilling. It's thumbing one's nose at a world that says "Because you can urinate standing up, you have to act like this and that and that and don't you DARE do that".
Maybe, most of all, it's like doing magic. If you found that you could transform yourself into a cat, wouldn't it be great? You'd pad around dark alleyways, climb trees, caterwaul, nap in sunbeams, maybe go to cat shows, just because you could. This is kind of similar.
A minor contributing motive is that, gender implications aside, the props of female dressing are kind of neat. It's interesting to smear pigmented goo on your face and achieve various visual effects. A flowing skirt is fun to wear, especially on a breezy day or if you spin around so it billows out.
The final motive is hard to explain, and it may perhaps be all in my imagination. There seems to be a dynamic at work whenever a male interacts with other people that involves overtones of power/status/ territory/potential for violence/etc. Do you know what I mean? It doesn't matter how harmless or unagressive you try to act yourself, you can't escape a certain sizing-up and staking-out. You may never have noticed it, but when I go out and people believe I'm a woman and act toward me that way, I really notice its absence. All at once, I'm completely removed from the subtle little power games that otherwise seem to be always going on to some degree. It's extremely refreshing, like clean, fresh air when you're used to smog. It may be that there's some competitive dynamic that replaces it for females, but I don't perceive it, so I can interact with people in an atmosphere that just seems really healthy. This is very hard to explain. You'll just have to trust me.
One thing that I can say is not a motive: I am not "expressing my feminine side" (a talk-show cliche). I don't have a masculine side or a feminine side. I am a single person, and if They want to label parts as 'masculine' or 'feminine', that's Their problem. There are those who feel like they need to dress up to get away from acting manly-manly-burp. Well, OK; but that's not me.