When you buy a computer, you usually need to buy a computer case in order to take the computer with you whenever you go on a trip or need to take your computer with you. You can’t just buy any computer case. Computers come in all different sizes and shapes.
Therefore, the computer and its cord which is the power supply must be able to fit nicely into the case. If it doesn’t, you will have trouble traveling with it. Another thing to consider is the number of internal drive locations. Sometimes, you have to plug things into the sides of a laptop and they usually stick out a little bit. These need to be able to fit into the laptop and with those in it, they should be able to fit into the computer case.
The size of the monitor does not have any bearing on the type of case. The only thing the monitor might have to do with the case is matching the colors. The number of LED indicators is also superficial. Similarly, the company that manufactured the motherboard will not have any effect on the case chosen.
This leaves the only two answers that make sense: the size of the motherboard and power supply as well as the number of drive locations. The bigger the motherboard, the bigger the case must be and more drive locations require more openings in the case. Other than these two things, choosing a case is cosmetic.