It's entirely possible to make a single ended amp that is as good as a balanced one, balanced isn't necessary in that regard. But it's often an easy/relatively cheap way to add more power, at a given pricepoint, or with a particular design. Particularly with small USB or battery powered stuff.

The person I was responding to doubted anyone "can actually hear the difference between balanced and single ended". But it provides in these type of devices typically 3-4x the power, and that's a very obvious difference, if you have something that needs it. HD660S doesn't need it.