Nf3 defends the center. It's just a natural move in the french. After the h4 h5 plan by white sometimes white can challenge a knight on f5 by playing nh4.
(i suppose the question is about Qa4?)
I'm not a french winaver expert at all, but i suppose one of the points might be to rule out a4-Ba3 ideas (one of the few ways to bring white's bad DSB into play). There must be subtler ideas behind the move, but you'll have to ask a winaver player. It's worth noting that such slow moves are possible because of the very closed nature of the position; it would certainly be suicidal to play multiple queen moves in the opening in most lines after 1.e4 e5, but in the closed lines of the french falling behind in development is rarely a terminal problem.
Does it somehow prevent the b6-Ba6 before black plays c4? That might be it.
EDIT: The idea of Qa4 is that c2 is very hard to defend, forcing the queen to defend (or forcing white to gambit a pawn), and prepares an a5-b5-b4 breakthrough.
As one who adores the Winawer, I offer you this explanation:
One of White's strongest plans in the Winawer is to gain space on the queenside by pushing his a-pawn and putting his c1-bishop on a3. Qa4 is aimed at preventing/delaying this, and White is left with a passive bishop pair.
Thank you all guys for your help!