Rolling is an important part of training and is done at the beginning of most Ninjutsu classes both at Bujinkan Reading and many other Bujinkan dojos. It is also common in other martial arts that do a lot of grappling such as Judo, but why do we do it?
For a start, it is a good way to warm up and get the body moving. Warming up is important for preventing injury. Rolling is generally done at the end of the warmup as it works the whole body.
Junan taiso, (body conditioning) rolling is an important way to condition the body so that it can handle the rigors of training and combat. We need to, in part replicate the stresses that the body and mind will face in a fight, without this you can very quickly be overwhelmed physically and mentally. We need to train ourselves to be able to hit the ground in a controlled manner and enabling us to either regain or maintain the initiative.
A common criticism of Ninjutsu is that the person who is having a technique done on them is going with it. This is to an extent true but is also a miss understanding of what is going on. Many of the techniques we teach are designed to break bones, dislocate joints, or worse. If you resist such a technique and it is done properly and with intent, this will result in serious injury. Often the only way to avoid this is to go with the technique and is in effect a counter to it. This is an essential part of our training; we are learning how to avoid getting hurt in a real fight. Obviously, a good practitioner in training will feel if the opponent is not going with the technique and will stop before they are injured, this too is an important part of training and demonstrates control.