By using the massed fire offered by a repeating crossbow, a commander can effectively nullify to his counterpart the use of his infantry, many of which are unarmored and thus would be weak to even the weaker force of these bolts. If all the enemy has to rely on is his knights, then you've forced you enemy into using a one-dimensional force that can be easily countered.
"Massed fire". What do you think an elaborate piece of equipment like this would cost? How many artisans do you have that can produce work like this? How quickly would a complex mechanism like this break down in muddy field conditions?