The computer case, also called the PC case, is the business card of your computer and essential when building a new PC. When assembling a PC, the case determines the type of motherboard it can hold, how many expansion slots are available, and how many hard drives you can install. Computer cases come in various sizes and are meant for various motherboard types. There are small cases that are easy to hide and large cabinets for the gamers, including brightly-lit RGB lighting.