Playing the Queen's Indian. Which kind of players? In general, the Queen's Indian is not much seen at top levels when arising from the ECO move order:

    [FEN ""]
    1. d4 Nf6 2. c4 e6 3. Nf3 b6


However, many games transpose from the Nimzo-Indian defense:

    [FEN ""]
    1. d4 Nf6 2. c4 e6 3. Nc3 Bb4


When at some point black plays b6 in order to fianchetto his light squared bishop. Now, these two openings have different basic plans and different kinds of players are adept at them: the Nimzo-Indian is known to be a reliable weapon against 1.d4 and the Queen's Indian is thought to be a little too passive.

The doubts I have are:


Which kind of players like switching between those two defences?
What is the goal of said transposition? (e.g. why not follow the main line?)
Why are such different conceptual openings usually transposed, when in certain lines not even the plans or pawn structure is the same?
Many Nimzo players already have the Queen's Indian Defense on their repertoire to deal with 3. Nf3. Such players may transpose between the two defenses whenever they have the opportunity and feel like switching.

To be more precise, it is after 4. Nf3 that black has the option to enter a QID/Nimzo hybrid with 4... b6.

    [StartPly "8"]

    [FEN ""]
    1. d4 Nf6 2. c4 e6 3. Nc3 Bb4 4. Nf3 b6


Nimzo-indian set-ups with a black b6 without a white Nf3 are still classified as Nimzo-Indians.

A reason to play b6 in a purely  Nimzo-Indian position is that it usually leads to slower games where you can outmaneuver your opponent in the long run. A famous example is Garry Kasparov vs Vladimir Kramnik (2001), where black had to win to level the match.

      [FEN ""]
      [Event "Botvinnik Mem Rapid Match"]
      [Site "Moscow RUS"]
      [Date "2001.12.08"]
      [EventDate "2001.12.08"]
      [Round "6"]
      [Result "0-1"]
      [White "Garry Kasparov"]
      [Black "Vladimir Kramnik"]
      [ECO "E44"]
      [WhiteElo "2838"]
      [BlackElo "2809"]
      [PlyCount "104"]

      1. d4 Nf6 2. c4 e6 3. Nc3 Bb4 4. e3 b6 5. Nge2 Bb7 6. a3 Be7
      7. d5 O-O 8. Ng3 Re8 9. Be2 Bf8 10. e4 d6 11. O-O Nbd7 12. Be3
      c6 13. Qd2 Rc8 14. dxe6 fxe6 15. f4 Qe7 16. Rad1 Rcd8 17. Kh1
      Qf7 18. Qc2 Kh8 19. b4 e5 20. Nf5 d5 21. exd5 cxd5 22. Nb5 Qg6
      23. Bf3 Rc8 24. Qb1 e4 25. Nh4 Qf7 26. Be2 a6 27. Nc3 dxc4
      28. Qb2 b5 29. Nf5 Nd5 30. Nxd5 Qxf5 31. Nc3 Nf6 32. h3 Rcd8
      33. Rxd8 Rxd8 34. Rd1 Rxd1+ 35. Bxd1 Qd7 36. Be2 Bc6 37. Qc1
      g6 38. Qg1 Bg7 39. Bd4 Kg8 40. Be5 Nd5 41. Bxg7 Qxg7 42. Nxd5
      Bxd5 43. Qc5 Qa1+ 44. Kh2 Bf7 45. Bg4 Kg7 46. Qc7 Qf6 47. Bd1
      h5 48. a4 Qd4 49. Bc2 e3 50. f5 e2 51. fxg6 e1=Q 52. Qxf7+ Kh6
      0-1


Vladimir Kraminik in New in Chess(2002/1), comment after 4... b6:


  At the board I decided to play the strange set-up that follows in order to keep all the pieces on the board and keep the pawn-structure flexible. Garry said after the game that he had a much better position. Well, it is clear that if you want to win against Garry Kasparov with black you have to make some concessions and play a worse position.