The CPU, or Central Processing Unit, is often referred to as the brain of the computer because it is responsible for executing instructions and processing data that drive the operation of all software and hardware components. The CPU performs calculations, makes decisions, and manages the flow of information involved in tasks ranging from simple calculations to complex computations. This central role in processing and coordinating operations is what earns it the title of "brain."
In contrast, while the hard drive is used for storage, holding all files and data permanently until needed, and memory (often referred to as RAM) temporarily stores data that the CPU needs while performing tasks, neither component has the same processing authority as the CPU. The power supply unit converts electrical energy for use by the computer but does not play a role in processing information or executing commands.