undeveloping in the French Defense

I recently played White in a game that began 1.d4 e6 2.e4 d5 3.Nc3 Nf6 4.e5 Ng8?! reaching this position:

Stockfish at depth 40 now prefers the move 5.a3 (and suggests the continuation 5...Bd7 6.Be3 b6 7.Nce2 c5 8.c3 += ).

I'm having trouble understanding the purpose of 5.a3, which runs contrary to my intuition, namely that after Black has undeveloped his knight, White should extend his lead in development by bringing out his pieces. After, say, 5.Nf3 (the move I played in the game), 5...Bb4 isn't any kind of threat and White has several strong responses.

So, why is 5.a3 so good?

While 5...Bb4 is indeed hardly a "threat", it does partially solve Black's problem of how to develop the kingside pieces harmoniously by intending to exchange one of them off (...Bxc3). Black's undeveloping 4th move intends to redeploy the knight to e7 where it will be in the way of the f8 bishop. So 5.a3 prevents this solution to Black's kingside problems. Very generally, all piece exchanges will benefit Black who is hoping to survive White's likely attack and head for the endgame, so White will do well to keep as many pieces on the board as possible.

Having said that, allowing ...Bb4 and just developing pieces quickly must be good for White too.

Oh. I guess Black wants to put the bishop anywhere other than d2, so you need to prevent a knight pin.