For one, it gives you 10, 10-minute meditations for free, so you get a sense of what the whole enterprise is about without laying down any cash. For some (many?), that's all you need to get going.

Second, having listened to quite a few guided meditations, I find the Headspace guy (I think his name is Andy) to be one of the best. The meditations strike a near-perfect balance of helping direct my energy and attention in a way that doesn't draw attention to himself or his voice/delivery. i.e. he gives excellent guidance and gets out of the way.