The main reason is protection.
The MCB protects the transformer in case of a short circuit. You don't actually need an MCB type fuse, any fuse type of correct size will work but due to the nature of Transformer and inrush, etc MCB is just easier to reset when trips.

The fuses for the drives protect each drive in case of short circuit or excess current draw. The transformer can supply many more amps than each drive can handle so if, for instance, motor shorts out the drive will keep dumping current up to it's rated output or until it destroys its self. some drives have built-in current protection but it's not advised to rely just on this, hence why a fuse.
Fitting a fuse same size or just slightly higher than the drives rated load will offer it some protection. In fact, I often fit fuses at a lower rating than the drives max rated current if the motors are not rated at or close to drives Max current.