Batgirl can be the edgy YA take on the Batman mythos (The Batman would've been more interesting if it had been The Batgirl imho) while Batwoman is essentially just an alternate Batman but she provides an opportunity to come up with a new origin story and background, plus even if she just played sidekick she's way cooler than lame ass Robin. Maybe not for their debut but it mite b cool if they showed up in an actual honest to goodness horror movie too. That's something neither Marvel (thankfully) nor DC have tried yet