Paper prototypes are ideal for the early stages of solution validation when teams are looking to test the use of solution ideas in a rough and scrappy way. Remember, paper prototypes should be adapted over and over and informed by user feedback.
Paper prototypes are intended to get user feedback quickly and without investing tons of upfront work. They are useful because they are changeable (think moving sticky notes around on a paper interface) and intended to be used, documented and thrown away. So don't get too attached!