The biggest lesson i got from annotating my game on the White side of an advanced Tarrasch French is the preservation of the light squared Bishop. I deviated from theory with 18. Bg4 rather than Bg6 and I wondered what is the difference.
Ultimately I believe it is harder for Black to exchange the light squared Bishop if it is not in its path after ... e5. This considerably eases Black's load. The model game is Zajick-Kousek, where one can see the extra lengths Black must go to exchange the light squared Bishop