you have to understand why fragging happens before you see the benefit.
Harddrives are split into clusters, default size is around 512bytes. When a file is put on the hard drive it is spread between as many contiguous clusters as it needs. However, when files are copies, moved, things are installed and deleted, some files can be spread between non-contiguous clusters (clusters that aren't next to each other) and this is called file fragmentation. Read times can be dramatically increased by fragmentation, but it has to be pretty badly fragmented to make a noticable impact on performance. Usually, the bigger the drive, the more fragmentation occurs, because there are often alot of empty clusters.
It depends on the level of fragmentation whether or not defragging will make a noticable difference.