Fans do not STOP mosquitoes, they slow them down.
Fans are better than nothing, but truly the rooms need screens on the windows, and screen doors.
1. The mosquitoes are not strong enough to fly into the blowing fan.
2. Mosquitoes are attracted to the breathing of humans, the carbon dioxide given off by humans is blown away or dispersed.
3. Mosquitoes are attracted to body heat, and sweat, the fan helps reduces the smells, and keep the body cooler.