Prior to the days of ABS, 4x4 did improve braking. With all 4 wheels interconnected up you could not lock one wheel up without locking up all. This means you could apply more pressure to the pedal and maximize the braking from the wheel(s) with the most traction.
With ABS preventing/limiting individual wheel lockup this is much less of an issue however I'd still bet having the wheels connected allows the ABS to operate a little less and less ABS = more braking.
The smoother you can slow the drivetrain, the less chance for wheel slip.
As for 4x4 usefulness, I use mine the most in the middle of summer to back trailers up my gravel driveway. Impossible to do in 4x2 when dry.
But like a couple others have said, if you have to ask...you probably won't miss it.