looper can be used to loop through through the frames of an arbitrary layer in a way not possible in after effects before or with types of layers for which looping is usually not possible at all. for example, you can usually not loop a nested composition. you have to create a quicktime move, import that one and tell it to loop. with looper you can loop the nested composition directly. also, with looper there are three different loop modes: normal, pingpong and random. normal loops through the frames and starts at the beginning when it is through, pingpong loops back and forth and random displays a different frame from the source layer in each frame.