How can a human figure out best move in this position? Why is 12. b4 played by a GM? Why doesn't he just capture the bishop? And how can he forsee so far in the future? Am I missing something?
Maybe before justifying the pawn sacs one should answer more convincingly why taking on c3 would not have pleased White. The answer is the backward pawn on c-file and future pressure against it by rooks from c-file. Black cannot be happier if the Queens get exchanged on c3 as well. So, taking the bishop back on c3 would have yielded nothing for White. b4 is half out of desperation as it is of creativity!

Augment to this the forcing sequence delineated by the other answers here, and you see that b4 is not as much of an abnormal move as it may look initially.