Italian game: Why is Nbd2 for white so good here?

This position is kinda idle, so almost any move, including Nbd2, is "good" here.

it's Giuoco Pianissimo: VERY calm/slow game. The idea is Re1-Nbd2-f1-g3/e3.

I also think this is the case. I play some Italian Game, though generally the Evans Gambit, and c3 - d4 is a common theme. 

EDIT: I also think it's worth noting that the c3 move gives the c4 bishop an escape square from ...b5-(Bb3)-a5-a6.

With Nc3, White makes a perfectly good developing move that, unfortunately, has no plan attached to it.  What is White going to do next?  How will he attack Black’s position?  The most natural pawn break is d4, but that’s hard to get and the Nc3 does nothing to prepare it.  After a d6 from Black, the position is perfectly symmetrical and White has a hard time proving any advantage.

With Nbd7, White is making a worse developing move as you point out: the Knight does not control d5, it blocks the Bishop and it has no obvious follow-up.  However, it leaves the c-pawn open, preparing for an eventual c3 and then d4.  Alternatively, White can play c3 and b4, playing more on the Queenside.  White will advance his pawns slowly and keep his pieces behind them.  White has a choice of plans, and notice how the position is no longer symmetrical, an important point in higher-level chess.

As for the Knight itself, it can go to c4 (usually after b4, so something like c3 and b4, chasing Black’s Bishop, and then Bb3 and Nc4) or g3, like in the Ruy (so Re1, then Nbd7  -> f1 -> g3 or e3).

i would move it to g3 so it could go to f5 and also limit black playing Nf6-h5-f4 and maybe black will play g6 and weaken himself

Thank you, this explains a lot.