As a les myself, though I wouldn't say I fit the mold of 'femme' or 'butch' (my hair is short because it's a nightmare to take care of long, but it's a gender-neutral cut that can get dressed up or down, and I own lots of guys clothes and I steal my brother's Old Spice in the shower but I still wear short skirts and go shoe shopping a lot) I'd say that the appeal has a lot to do with what makes you feel comfortable in your own skin, and also a lot to do with societal pressures (at least where I live, it's generally more acceptable to be lesbian if either your girlfriend or you looks like a boy; I don't get it, personally, but there's a lot about where I live that I don't get).

As for the girls that date them, whatever the packaging is like, inside is still a girl with a girl mind who acts in girl ways. Also, personally, androgyny is hot. And a (more femme) lesbian tends to want a lot of the same things a straight girl wants in a relationship; physical security, emotional support, etc., and personally it's a lot easier to find in a relationship with a butch than a relationship with another femme. At least, that's my two cents; having a grand total of two relationships in my life, I may not be as qualified to answer as others.