Because is what makes the difference between training in a form of budo and training in, for instance, basketball.
But why is budo training better? And why is this specific type of budo training -- training which seeks to prepare you for real fights -- better than less "realistic" budo training?
Closely related is the question of what "real" means, anyway. Which is more real? Full contact sparring with gloves and protective gear, or paired koryu sword kata with live blades?