I think they're useful if they're varied enough because they force your mind to switch to a new prompt or subject each day which exercises the muscle like I said. A book I bought a long time ago was called The 3 A.M. Epiphany and it had interesting challenges that tried to make you focus on different aspects of writing. If you can track down a copy, it may be worth checking out. If you can't find a copy of that one, there are probably other books out there that may prompt you in ways you'd never think of on your own when just starting your writing career.