It allows for fine RPM control. If you want fans that can quickly and easily adjust speed depending on system load you should get PWM fans. They're typically not significantly more expensive.
I suggest "Arctic P12 PWM PST" fans. They're quiet, have good performance and don't cost much. The PST feature allows you to chain multiple of them together using just one motherboard fan header.
12 minutes ago, Levent said:
allows fans to run at lower rpms
Non-PWM fans can also do that. But they have to be regulated through voltage. And not every mobo has "smart" fan control for DC fans, meaning more often than not they're running at fixed speeds and/or maxed out.