Why play Nb5 in this line of the sicilian? After 1. e4 c5 2. Nf3 Nc6 3. d4 cxd4 4. Nxd4 e5 why is 5. Nb5 the best move?
The game usually continues 5. Nb5 d6 6. N1c3 a6 7. Na3 and it looks to me like whites knight on a3 is misplaced. What makes this line good for white?



      [StartPly "8"]

      [FEN ""]
      1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 e5 5.Nb5 d6 6.N1c3 a6 7.Na3
The real reason why this is the main line lies in the many possible variations after 5.Nb5 and its alternatives. Any verbal explanation will at best be approximately correct. With that in mind, here is my take:

5.Nb5 is the variation that best keeps control of the weakened d5 square. If black achieves d5 its only strategic problem will vanish, so keeping control of d5 is whites main strategic objective. 5.Nb5 provokes d6, which makes it impossible for the Nc3 to be neutralised by Bb4. This Bb4 move is a problem if the knight retreats to f3 or b3. If the knight goes to f5, d5 is even immediately possible.