This is an unfortunate retreat for the White B, as it does nothing on e2. Exchanging on c6 was not advised, as this would have given Black a B pair against a N pair, which would have been easy to exploit in this open position.
Positional assessment at the transition from opening to mid-game: White has made seemingly normal developing moves, yet he has drifted almost imperceptibly into a slightly awkward position. White's main strength is his good grip on d4. However, the White Q on c1 is blocking the development of the QR. Assessment for Black: rather better prospects for controlling the center since the f- and e-pawns are mobile (whereas White's f - and c-pawns are blocked by his own pieces). Black's heavy pieces are massing on the half-open c-file, exerting great potential pressure against White's Q-side. The half-open g-file may become active as well to put pressure against White's k-position. In sum: Black has fully equalized already and has good prospects for increasing pressure in the center and on the g- and c-files; whereas White will have some difficulty completing his development.
12. Rf1d1 Bf8g7 13. a3
To prevent the Black N from coming to b4.
With this and the previous move, Black significantly increases his control of the center. Black has now more than equalized; the initiative, I think, has now passed to Black. White will be defending from here on.
White's main strength is the half-open d-file, so he prepares to pile up his heavy pieces there.
Immediately: preventing the White N from occupying a4 and driving the Black Q away from c5. For later: nailing down c4, a potential outpost for the Black N.