The CPU is the unit responsible for carrying out all instructions of an application. This includes how to run different subsystems that keep the OS running (multimedia, audio, render, etc.).
When the CPU usage is high, the user may experience sluggishness or higher battery usage (among some other symptoms):
Since the CPU usage is a shared resource, abusing it may prevent other services that are also running to work correctly, affecting the user experience as well as the proper functioning of Android (and the applications that run in parallel).
With a higher number of instructions, the CPU increases its speed with a consequent increase in use of voltage that causes the device battery to drain faster.