A computer case provides two things for a computer. First, it provides a standardized format for the installation of non-vendor-specific hardware. Secondly, it protects that hardware and helps to keep it cool.

As an Industry Standard

Cases help standardize the safe installation of different vendors’ hardware. Samsung and Western Digital each make hard drives with different capabilities, for example, but because they are a standard size they will fit equally well into the same 3.5-inch hard drive location (or bay) within a case. Such standardization allows for customization and lower production costs.

As Protection

The exposed circuitry of a motherboard can malfunction if it gets bumped, something spilled on it, or too much dust. Cases protect delicate internal components such as the motherboard from such danger. Additionally, case fans help air flow and prevent components from overheating.