This keeps them from closing the doors on Grooveshark, and then immediately starting some new service, say GrooveBarracuda (or selling their software, patents and IP to some other enterprise looking to do the same thing).
Since the record companies now own their IP, anybody who tries to resurrect Grooveshark using the old software would also face charges of patent infringement, trademark infringement, etc (unless they build everything from scratch, which would be a much larger investment).
For the record companies, this helps them avoid future legal battles, and lowers the threat of a similar service emerging.
For Grooveshark, maybe this gave them a better settlement (e.g. lower damages owed to the record labels).