The box jump is a plyometric exercise that offers a benefit that not a lot of other plyometric exercises offer: it's an explosive jump without the impact of landing. Therefore it can be used early in your plyometric training progressions.
The box jump is an explosive jump. It's about how much force you can put into the ground in order to jump as high as you can. Unfortunately, people often make it about the height of the box.
The problem with this approach is that it can be misleading, because you can jump onto a higher box by tucking your knees in. The result is, you jump onto a higher box without actually jumping higher.
Instead of focusing on the height of the box, focus on an explosive jump, where you land in the same position on the box as you jumped onto the box. The hip angle and knee angle of the landing should be the same as the hip and knee angle of the deepest position of the dip before you explode up.