Why is CPU needed?

The CPU (Central Processing Unit) is needed because it is the main part of the computer where all the calculating and control activity is done. Program instructions are decoded and executed by the CPU, data is manipulated in the CPU. Without it, the computer will not operate at all.
A Processor or CPU is essentially the computer's judgment and logical part of the brain. It performs the basic arithmetical, logical, and input/output operations of the system.
The CPU is the part of every computer that actually does all the work, all the other parts of the computer are only there to support the CPU's needs as it does that work.