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What exactly is the layer fan for? When should I run it and what materials should I run it on?

I use ABS so layer fan is not really necessary. But I sometimes use it for bridges or for printing small objects at high speeds.
If you print PLA it's a must.

415porto wrote:What exactly is the layer fan for? When should I run it and what materials should I run it on?

The layer fan is for cooling the top layer of the printed object. Generally speaking, you very rarely need it when printing ABS, because you want the most recent layer to stay as hot as possible before receiving the next layer (low differential reduces curling). However, it may be beneficial to use it when bridging or overhanging to prevent droop. Generally speaking, you usually use a layer fan when printing with PLA. PLA doesn't shrink/curl like ABS. However, leave the layer fan off for the first few layers of a PLA print. This will allow the object to develop a better bond to the build surface. Use of the layer fan and the speed at which it is run at will depend on the size of the object and the print speed. Small objects or high print speeds mean less time for a layer to cool before the next is put on top of it. There is a sweet spot for any print, where the last layer is just cool enough to be structurally sound, but still hot enough that the new layer can make a strong bond. Most slicing programs will provide some conditional fan control to help manage cooling.