8. ... Be6
Although Black has a solid queenside and central position (the b7-c6-d5 pawn chain), he is quite behind in development. It will not be long before the Bishop on b4 is forced to retreat, losing more time. Hence, Black must develop quickly before the Bishop is forced back.
The key tactical problem for Black is the K-side. The KN is pinned and the castled King is somewhat vulnerable because there are not enough pieces around to defend him (White can bring his Q, B and KN to bear on the K-side faster than Black's pieces can rally to defend it). Also, should Black leave the KN unprotected, BxN could open up the K-side.
Black has two options to defend the KN:
(1) Be7 - Played right away, this move loses a tempo and Black lags further in development.
(2) Nd7 - Played right away, this move blocks in the QB and the development problem remains.
Black now has two other candidate moves left to strenghten the K-side:
(3) Re8 - Takes the open file. However, this does not solve the problem of the pinned Knight.
(4) Be6 - Strictly speaking, this move is not needed to defend the d5 pawn (White still needs two moves before he can attack the head of the pawn chain with c4). However, it does free the QN to take its preferred position d7. Hence, Be6 was played as the least of all evils in this position.
The only catch with Be6 is that it is vulnerable to exchange (after Nf4) - giving White the Two Bishops advantage. However, since it is a "bad" bishop (behind a pawn chain of pawns on the same coloured squares and hence hemmed in by the chain) exchanging it for a more active White piece (like the Knight) gives Black some reprieve on the K-side and moves towards an eventual end-game where both sides are more equally matched. Also, the current game position does not have significant open lines for White's bishops to play with - and once the pin is broken, Black's Knights should hold a [very small] edge.
Just to put things in perspective here's my analysis on the game:
C01: French: Exchange Variation
1. e3 e5 2. Nc3 Nf6 3. d4 exd4 4. exd4 d5 5. Bg5 Bb4 6. Bd3 0-0 7. Nge2 c6 (last book move) 8. 0-0 (Position is nearly equal, with Black having a small advantage)
And my last move...marks the end of the opening book and the beginning of actual thinking.
9. ... Re8
As I mentioned in the previous post, I am willing to conceed the two bishops to White to relieve some pressure on the K-side. Re8 recaptures the Bishop without leaving a backward pawn on the open e-file and also provides additional protection to the KN. Of course, the rook can be forced back by his bishops, but I hope to play Nd7 before that.