I am working through the book mentioned and in game 8 (Przepiorka vs Prokes) at move 4 ...P-B4, which is black moves pawn to C5. The book states the following: "This move is almost indispensable in Queen Pawn openings. It is important not to play ...Kt-C6 first, as the bishop pawn must not be obstructed." Could someone shed some light on the importance of ensuring the queen's bishop pawn is unobstructed?
Often black plays b6/Bb7/c5 to support an attack against the White d-pawn, and at times even Na6 to support c5. Obstructing the c-pawn slows down or prevents these kinds of maneuvers. Nc6 is an awkward placement of the knight on the queenside with d5/e6 being played by black, as it has no real good second move from c6 to get out of the c-pawns way, thus losing time, if black wants to follow up with ...c6 or ...c5. The move ...c5 is a "breaking" move that black wants in many openings, including the Caro-Kann, the French, etc., and not just d-pawn openings. A knight on c6 is generally speaking a 'good' knight when ...e5 and/or ...c5 has been played, i.e., king's pawn openings, The Sicilian Defense, etc.
I've often thought that the author over-emphasized this point. But there are good reasons for ...c5 in Queen's pawn openings. This single move accomplishes three things. 1. Put pressure on d4, as @Jerry Snitselaar has said. 2. Make room for the Queen Bishop, assuming ...e5 is not workable. 3. Create a meaningful open file for a rook--again, assuming ...e5 cannot reasonably be played. (I say "meaningful" because it is relatively easy to open some file, such as with the a- or b-pawn, but usually a rook on the c-file hits more valuable targets, and besides, it is usually important to prevent White dominance on this file.)