Why? There is no exploit; to the device, this behaves as any other Android app, subject to its restrictions.

This is indeed very useful, as not all phones may be rooted (an issue of legal permission between owner and user, not of technical possibility), and this app enables a pseudo-root, sandboxed system that can be seamlessly uninstalled like any other Android app.

Also, the attack surface of a rooted phone is siginificantly larger (by necessity) than that of an unrooted one; plus you may want to give your users a portable desktop computer without necessarily giving them a rooted device.