Why is Bd3 a good move?

[IMG]http://i.imgur.com/XXOFAh7.jpg[/IMG]

1. e4 e6

2. d4 d5

3. Nc3 Bb4

4. exd5 exd5

5. Bd3 Nf6

I don't get it. Bd3 isn't defending anyone or anything and it isn't attacking anyone or anything. I could play Bb5 in the next move, but what's the use? The King would simply move to f8 and the black Queen would knock the white Bishop out.

this position is still the opening phase of the game so you don't need to be attacking anything. Bd3 is a simple development move that prepares short castle and puts long-term pressure on h7 (for once the black king castles).

if you play 5. Bb5? I imagine 5. ..c6 would be played forcing your bishop to the side of the board where it could be attacked by pawns. one simple rule of chess is to never start an attack until all your pieces are developed

opening -> develop your pieces to good squared and castle.
middle game -> formulate a plan of attack.
end game -> win

Thank you very much, Alieksandr, that was really helpful. :)

look the classic bishop sacrifice in the french defence and allied openings............ Bd3 is a very powerful move. it has many tactical chances.... i know it personally since it is my main tactical weapon in king and queen pawn openings.... look Marshall attack also............ learn openings relating it with a trap is very beneficial and will be more fun......