Depends on the size of the incubator. I've hatched hundreds of eggs in small hovabator style incubators without fans. In some cases a fan can be a detriment in that it can more quickly dry out the environment. In large incubators that hold more then a clutch or two of eggs, a fan is important to help equalize the temps throughout the whole enclosure so you don't have any hot spots or cold spots. Small incubators with one or two clutches of eggs don't really need a fan.

Mark