All answers and suggestions are almost correct. I will tell u in easier way ...
When you drive in salt range you DO NOT need S or B unless its raining heavy and road is slippery then you should engage B. B is for brake mode. You will feel engine holding back your gear ratio to produce more power in uphill and downhill terrane. While S is for sports mode it delays the gear changing ratio hence resulting in more pick up in each gear.
An ideal place where you should use B is if you are going to Shogran ..... even the track to Pir Suhawa is not hard for normal D drive.
Best way is to experience it by yourself, try switching between D S and B but not on very high speed. You will feel the difference.
I engage B in mud, sand, very rough terrane and my belta (1300) crawls easily with a gentle touch of acceleration.