Why take C6 bishop with D7 pawn rather than B7

Good topic. There is a mate threat if black plays 3...Bc5 then 4. Nxc6 dxc 5 Nxe5 Qd4 and black is threating mate or picking up the undefended knight. There are options after this but I've scored good with this line (at my low level).

@petri999 world champion lasker played b takes c in the berlin.

@tpr what do you mean with unoposed, white plays d4 in both lines.

@Dario19503 I already see FBI hiring you soon.

Thanks guys :)

@ioanis
Yeah, generally speaking, when a move (like dc6) violates a certain principle, it is usually either:
1) because the other general principle was more important to follow
or
2) because the violation is tactically justified

In this case, it's both - 1) it makes the position more open for the bishop pair and 2) e-pawn is indirectly protected with ...Qd4.

#16
It is 1) only. After ...bxc6 e5 is indirectly protected with ...Qe7.
Besides ...dxc6 not only opens the diagonal for Bc8, but also the d-file for Qd8 and it keeps one island of pawns a6-b7-c7-c6 instead of 2: a6-c7-c6-d7, i.e. it avoids the a6 pawn to become isolated.
...bxc6 is bad because white can play d4 and on exd4 Qxd4 white gets a centralised queen.
After ...dxc6 d4 exd4 Qxd4 Qxd4 Nxd4 it is only a centralised knight and it can be chased with ...c5