For the same reason you do a clean install once in a while. To get rid of things that accumulated over the time, slow your system down. To question all the dozens of tools that you downloaded and installed and actually don’t need anymore. I always take this opportunity to only install what I really really need and use. Get rid of things that might slow down my system.
So I’m picky about what I transfer over. And if I don’t take everything over, I need to hunt for the things I want individually. And everything making this process easier is welcome. Saving all user data and prefs in a profile that’s easily exportable, or saving it all in the user’s document folder so I can just copy my home folder would be great. On the Mac, many settings are in multiple hidden settings and prefs folders, which is annoying.
@MattiasNYC, thanks for the reference!