Why does Black play 10...a6 in the A39 variation of the English opening? I observed that after 

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1.c4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 g6 6.g3 Bg7 7.Bg2 O-O 8.O-O Nxd4 9.Qxd4 d6 10.Qd3


In many games, Black played the strange move 10...a6!? It has been played by top players like Kasparov, Anand and Carlsen. 

What is the point of 10...a6? Is it a waiting move for White to reveal his hand? It seems like a preparation for a future b5, but why at this particular moment when Black could simply develop the bishop on f5 (10...Bf5)? Also, in some variations, the pawn could be useful on a5 so that Black's knight can rest comfortably on c5, so 10...a6 could be a waste of a move.
This position is discussed in many books, the consensus being that 10... a6 is the best and most flexible move. As you say, black waits for white to reveal their hand, and a6 is generally useful. Play could continue 10... a6 11. Be3 Bf5 or 10... a6 11. Bd2 Rb8.

The move 10... Bf5 is a serious alternative since it provokes 11. e4, blocking the diagonal for white's bishop. In this variation white would ideally like to hold back e4 to keep the diagonal open. Still, white can keep an advantage in various ways. Marin in Grandmaster Repertoire 5 says that white should try to emulate Smyslov-Timman, Moscow 1981.

White often chooses a set-up with Bd2, b3 and Rac1, leaving black's Bg7 bishop with nothing to attack. In such a position black needs the b5 break to create active play. Therefore playing a5 is usually not a good idea.