Defragment when no fragments?

Hey, as we know Defraggler comes with options to defragment fragmented files, but there's also option to "generally" defrag the whole drive. This one got me puzzled. How does Defraggler determine which files should be close together in hard disc? Or if this heuristic is lacking, is there any benefit to run full defrag when only a few files are fragmented since last use? I noticed that whenever I play a particular game that game's files end up fragmented.

Thanks.