The Process of Decision Making in Chess Volume 2: Practice positions and solutions. Position 9.8

Solve for white, address the challenge below:

Diagram 9.8- in this position arising from the scotch gambit, observation shows that black has a set of backward double c pawns. Though currently it’s clear that white will not win any of them, at least in the imaginable future, their weakness and backward structure create great opportunities for white to fight for the advantage; the fact that they are backward (cannot potentially be supported with any other pawn) means that if blockaded, they stay blockaded. It also means that if they’re attacked they must be protected by pieces.
From that observation we can formulate an idea. We will use the fact that the c pawns are blockaded to dominate the dark squares, and we will do so with initiative while attacking c6: Qc3! With tempo, followed by Bc5 and possibly a future Nd4. The backward double pawns structure allows us to dominate the dark squares with no possibility of contest by black, and we create progress with initiative while forcing black to respond with moves that only defend the weak pawns (such as Qd7 in response to Qc3).

Challenge: part of the suggested plan was to play Bc5 after Qc3; can you find the reason why this is better than Nc5 after Qc3?

Only thing I can see quickly is that it enables the capture of the c6 pawn. I will look a little longer.

By the pin on the knight for bishop.

so white can leave the d4 square for the night

Because taking with the bishop attacks the rook and forces the exchange of bishops and also because the knight is needed for a final assault on the pawn at c6 with an eventual Nd4.