A Saiyans Tail gives him the power to turn oozara when theres a full moon.But if there tail is cut then they cannot change.A tail can be regrown if it is cut and will always grow back.The tail could be said a weak point of a saiyan and when it is grabbed hard the saiyan will be paralyzed untill the grabber lets go.
The only way a saiyan can lose his tail forever is to cut off there but.A saiyan's tail is never gone cause if you were to look at super saiyan 4 Goku his tail is there.