Or having the time of your live doing it !!!!
A fig file is basically a biped preset.{structure file, 5 spine links, 3 finger links etc}
Say you have made a custom biped for apollo and rigged him, you can then save your biped as a fig file.
 So when you make an other character like apollo you load your fig file and the biped is created for you.
Another plus of fig files is loading mocap on it, when loading for example a bip,bvh etc you specify your fig file and the mocap is transfered to your bipedstructure.