As a WMD and a beacon of power. The ability to destroy a planet would have given him great power - everybody would've been afraid to touch him.I have a feeling he just wanted it because Kirk had it and that was enough. I can't really see him trying to sell it on EBAY or something. Just a footnote though...this is one of my favorite movies of ALL time.
The simple answer is anything he wanted if he hadn't been stopped. Khan was a man who desired power above all else and with the Genesis device he could of held an entire planet hostage. Remember this device had the ability destroy all life on a planet where life already existed.
Perhaps Khan would have teamed up with Kruge