What is the point of ...Nbd7 in the Queen's Gambit declined?

I'm not a 1.d4 type of player. I've noticed that, in the QGD, black usually plays their knight from b8 to d7 instead of the more natural c6.

I do understand that in some variations there is a pawn in c6, therefore it'd be impossible to develop the knight to that square. But in some other variations the c6 square isn't taken and still black prefers to play the odd-looking ...Nbd7.

What is the idea of moving the knight to that square? From there the knight can't control as many central squares as from c6 (and this is a fundamental chess idea in the opening). I'm a beginner though.

Thank you.

In d4 d5 games it's not natural to block the c pawn with a knight because it's either used to support the center (c3/c6) or attack the center (c4/c5). We can't say the same for e4 e5 openings because the board is not symmetric in that way i.e. moving the f pawn weakens the king.

(an early Nc6 or Nc3 isn't terrible, but it's uncommon)

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But if you mean after playing c5 why would the knight go to d7, sometimes that's the better square because it keeps the c file unblocked.

You can also pay attention to the advanced squares a knight might have on c6. In other words if b4, d4, and e5 are firmly under the opponent's control, then a knight on c6 may just be a spectator.

And I guess I should add to my post that if black has successfully played c5 already, then developing the knight to c6 is often fine.

As an easy example, in the Tarrasch (shown below) I assume the knight usually goes to c6.

So maybe as a general rule we can say if you've already played c5 successfully, and if there are no tactics like @inkspirit talked about (the c6 knight gets into trouble) then Nc6 is fine.

I think Tarrash is not a very good choice for black.

Black is tying to open the centre prematurly ( without proper development) that brings more opportunity( advantage) for white. 

Do you actually think that, or did the engine on your phone not understand the position on move 3, so you decided you don't like it either 

As the beginner that I am, I would be a little bit scared of playing the variation you showed with black, since White can force you an isolated pawn on d5. Wouldn't this be a permanent weakness?

Isolated Pawns have both strong and weak points. 

The strong points include free development, good command of space, two good Knight outposts on the open files (e4 and c4 in the Tarrasch) and also White's reluctance to exchange a Knight that occupies the outpost, since that exchange would rectify Black's Pawn structure.

Not a huge fan of d4 opening, but as far as I know that it's for letting room for c pawn push .

Yeah, but it's a classic battle of worse structure compensated by active pieces.

Sure most endgames are difficult to draw for the IQP, but the great majority of them are defensible, and before the endgame comes the middlegame, and the IQP often affords some nice attacking chances.

In the end it's not bad or good, it's just an option.

The Tarrasch may not be the defense of choice for top 10 players (it's not ) but lots of GMs have played it, and in general the isolated queen's pawn is nothing to fear.

It is just my common sense. As an example, Carlsen can do useless trash like 1. Nf3, Ng1 or K and Q swap multiple times cos the centre is closed. He cant play like that when the centre is open. In open centre, the player who has extra turn get more bonus! (usually white) 

And also most sub optimal opening are playable when centre is closed! 

Of course , I lose much interest in human games to learn. For example, Famous GOD like games of Paul Mauphy that I learned as my beginner turned out to the feeling now that  there is not much point in learning where people do serious mistakes here and there. (most of his opponents dont even know how to develop pieces, like 1300 players).

Ok but stuff like the french exchange and slav exchange are notoriously boring / drawish even though the center is opened very early.

White's best line against the Tarrasch is NOT to fully open the center anyway. Why would it be? He has no advantage in development on move 3.

Not a fan of the line you gave.