You don't "need" to exchange your dark-squared bishop, but it is definitely a great option to consider. A "standard" Benoni play would consist of some sort of ...d6 and ...e6 play on the center, but White is probably going to be comfortable there and has many options available.
Another option available to Black is to counter White's space advantage by playing an immediate ...e5, but this would make the g7 bishop a horrible piece, that's why trading it on c3 may not be the worst of ideas. In fact, 4...Bxc3 5.bxc3 e5 is a quite successful continuation for Black.
Whether Black opts for an ...e5 push or not, Hauke Redmann's points still apply: another benefit of trading the bishop for the knight is the damage you cause on White's pawn structure. Black can create pressure on the queenside by attacking the weak enemy "c" pawns, but it's not always easy to capitalize on that (there's no clear plan to coordinate Black's minor pieces for an attack on those pawns).
As a final note, when discussing openings, please take into account that "it takes two to tango". White can't play a Queen's Gambit unless Blacks allows him to. The position we're talking about is a Benoni. Not a Benoni by Black against White's Queen's Gambit, just a Benoni. And it wouldn't have been so had White not played their pawn to d5