1. Black needs the d-file to get to g2
2. Lining up bishop and queen against g2
According to Michael Adams in Think like a Super-GM, the attacking potential of the combination of opposite colour bishops and queens is insufficiently appreciated below a certain level. The non-master associates opposite colour bishops with drawing chances in the endgame. But ally the unopposed bishop with a queen and their combined power on one colour complex, with no corresponding bishop to contest it, provides attacking possibilities. Generally, as observed by Dvoretsky in his book Positional Play, whichever side first takes the initiative wins. But to first take the initiative often requires pawn sacrifices.
In position 1 (Praczukowski - Williamson, 2014), black needs access to d2 for his heavy pieces in order to further attack g2, so sacrificed his d6 pawn with 21... c5!.
Later in the game, in position 2, black won by replacing his bishop on e4 with the queen, enabling a fatal double attack on the weaknesses at e3 and g2.
Note: The same principle can also apply to opposite colour bishops and rooks.
See: 'Initiative'.