Hi ElMatador,
Welcome!
It depends what photos you have been looking at. Fans are used to cool the 'cold-end', (the upper part of the extruder) keeping it cool. Be careful not to let air cool the hot end though if you are designing your own.
If you see a fan cooling the printed work the it's probably a small piece that's being printed. A concentration of heat from the heater block when printing a small item can deminish print quality, hence cooling the small piece with a fan can help print quality.
You don't need a fan specifically for bridging. The software can be tuned for bridging (within Skeinforge for instance). The filament is stretched across the gap, and shrinks slightly as it cools pulling the bridged filament tight.
Hope this helps a little. Keep asking questions and enjoy your project!
NumberSix
[
numbersixreprap.blogspot.com]