Maybe I'm naive since I'm a relatively new PC user gone mac- but why would you do this?
If this was in Windows, the only conceivable reason I see to do this would be for a super defrag (which could be done not only better, but optimized, with an aftermarket defrag). There would be absolutely no point in doing this to try and heal a "heavily used" windows installation to rid it of unnecessary clutter and junk lingering from months or years of use.
Now, from my understanding, there's no real point to defraging OS X as it defrags on the fly or what not. Since you're making an exact copy, and copying it back, you're not going to get rid of clutter- so what's the point of all of this?
The only other thing I can think of would be to zero out the drive for security as it's somewhat pointless, from a performance point of view, to zero it out then not put a clean OS install on it.