Is motherboard fan necessary?
The motherboard needs cooling. Cooling the motherboard can prevent damage to the hardware. A motherboard that is too hot can damage the components. Keeping it cool can increase computer lifespan.
Why do motherboards have fans?
The fan is for the x570 chipset. Due to x570 supporting pci4 (among other factors) the x570 chipset gets hot and needs active cooling with a fan. The fan can be removed, but it will cause the chipset to throttle and is overall a very bad thing to do. There is a reason the engineers put the fan there.
Is it safe to run a PC without fans?
Completely safe as long as you keep an eye on your Motherboard Temps.