It's important to play 6...b5 now because after 6.Nc3(or Re1 or d3) the pawn on e4 is protected so White is thretening Bxc6 and Nxe5. After 7.Bb3 the most accurate is 7...0-0,forcing White to play 8.d3 before playing Nd5,which could otherwise,say after 7...d6,be played as a pawn sac.
Well,I agree,but apparently it gives White an extra possibility:8.a4 b4 9.Nd5 Na5(9...Nxe4 10.d4 is supposed to be good for White) 10.Ba2 Nxd5 11Bxd5 c6 12.Ba2 c5 13.c3 Rb8 14.d4! White saves a move(d3) This is from Mihail Marin.