What is the difference between a 6 wheel swerve and a 4 wheel swerve?

[Ether]

Originally Posted by Nick Lawrence

If the wheels are not steered independently, (which I don't remember them being on 1625's 2010 robot) then it allows the robot to rotate about its center much easier, with far less turning scrub than a 4WD crab setup.

That part makes sense.

But if the wheels are not steered independently AND you have drop-center wheels, of what use is the steering? (Unless the center wheels are omni or low traction?? In which case, what's the point?)