I don't have an extensive knowledge of the opening theory of the two openings mentioned in the title. However, I do understand that, at a very basic level, in response to the Queen's Gambit the moves 2...c6 and 2...e6 have the effect of defending the d5 pawn as well as erecting a solid position for Black.
However, the move 2...c6 has a simple advantage: Black's queen bishop is more easily developed.
Why, then, is the move 2...e6 even played? What advantages does it have over 2...c6? It seems to me as though the move just impedes the development of a piece.
The same consideration applies to semi-slav type structures in which the queen bishop is still on c8. Why would one want to play e6 before playing Bg4 or Bf5?