Why Defragment ? And the answer is...

Here's what my drive looked like after "refreshing" (re-installing) Windows 8 and then adding 94 updates from Windows Updates, plus several of my third-party programs. The little red squares are fragmented sectors, and there's lots!

And here it is after Auslogics Disk Defrag defragmented 1260 files. And that's why you need to do this - so the various parts of a program can find each other in a hurry when you ask it to do something.