Achieving a uniform, and specific sized curd cube can be considered and important part of cheesemaking.
Curd size influences the amount and rate at which whey is released from the gel. This in turn does other things and affects the resulting cheese.
Some recipes call for very specific curd sizes.
A boning knife will of course work, but achieving a specific sized curd is easier, more reliable, and in many cases, more practical with a knife that has specifically spaced wire blades.
For example, achieving a horizontal cut, that is evenly spaced, under the surface of your coagulated milk, is difficult, if not impossible with a boning knife.
That is just my 2 cents.