I like it because it makes me feel like I'm "being honest and true to myself" (if I can put it like that). I just really, really, don't like thinking of myself as 100% male. I feel have have a strong feminine presence within me and I want that, I like that, and I need to express that. Dressing 100% male is not something that makes me feel comfortable because it seems like I'm somehow "betraying" my inner true self. I'd like to dress fully more often but by under-dressing it gives me a little "femme hit" (if I can put it like that) and thus makes me feel closer to the person I'm supposed to be.
I always wear minimally a bra and panties (even at work) but depending on what time of year it is (it gets hot in the Sacramento valley) I sometimes add a camisole and panty hose too. Incidentally, this is the same reason I also wear fairly girly clothes on the outside too in my (girly) boy mode