It's defrag, Jim, but not as we know it.

http://www.hanselman.com/blog/TheRealAndCompleteStoryDoesWindowsDefragmentYourSSD.aspx

(long, technical and interesting. tl;dr: SSDs aren't immune to fragmentation, but that word does not mean what you think it means in this context, and Windows does The Right Thing when you just leave it the fuck alone. Unsurprisingly, the filesystem engineers at MS know more about this shit that a bunch of neckbeards on the internet)