Why does a cpu fan need to be automatic?

It IS on at all times, it just runs faster or slower depending on the workload of the CPU and resulting temperature change. You could set it to run 100% at all times, but it doesn't need to if the CPU is idle.
 
Hi newcomer and welcome to the Tom's hardware forum.

The CPU's fan always is on, the only difference is that when your CPU is in idle, the fan is working in a low rpm, when your CPU is in load, the CPU's fan works to high rpm.