Why are interrupt required?

Interrupts are required in order to get the attention of the CPU. A CPU typically has two interrupt lines. One is the nonmaskable interrupt line (NMI). That is used in the case of critical errors, since this interrupt cannot be ignored. The other one is the regular interrupt line. That is used by hardware devices and certain software to get the attention of the CPU. When you move a mouse, for instance, that creates both a hardware and software interrupt. So the CPU would then process the mouse driver code and move the cursor, then get back to what it was doing.