Various superficial ideas - increases protection of d6 via Qd6 and Bd6, discourages e5 by white, enforces ...e5 by black, develops the c8-bishop, gives room for ...Qd7/Bd7 (eyeing ...b5), increases black's space for development...
Many ideas. Of course all meaningless out of context. ...d6 usually is played in the Dragon, Scheveningen and a bunch of other lines in the Sicilian. Look up relevant theory to try and understand more in context.
I Have been playing sicilian for 3 months now and i really find it preity good for black! but the only thing i dint get about sicilian is why D6 is played! it doesn't control the center as d5 does..! maybe its played to control e5 square or to defend the c5 pawn, but c5 pawn is normally traded for the white's D pawn! so plzz suggest something!