If you banned firearms a lot of people would hide them and never claim to have had any.
If you require registration and told people guns would never be banned, most will register rather than become criminals, and then later when and if you ban guns it makes them a lot easier to round up.
Or you could control them in other ways like Chicago did. They required registration, and then they made it illegal to transfer a registered handgun to anyone else and illegal to register new ones. Basically, it was a ban that resulted in handguns totally banned as soon as all the owners died. Or if registration is free now they can impose fees later on to generate revenue.
Kind of like how they put red light cameras up in states other then California even though no bill has passed authorizing their use in such states. They sit ready, just waiting for the legislation to pass to make them useful.
Of course, now that Heller has made Chicago's ban void, handgun registration is a lot less useful, but even if they can't ban them outright they can use the registration to make sales more difficult and apply other restrictions to guns. The name of the game is to make gun ownership and use as hard as possible. The Feds still maintain a ban on all machine guns made after 1986. They simply required registration and made it illegal to register any more.
Didn't California already use "assault weapon" registration to confiscate certain "assault weapons"?
As for actual benefits for society, just like for all gun control, there aren't any actual benefits... unless if making anti-gun people irrationally feel cozy is a benefit.