Why should I try to put knights on weak squares when they can be taken easily? Here you can see two weak squares-b6 &amp; d6. That's what my chess book says, and in order to use these two weak squares, one must consider the moves Nf3-d2-c4.

Actually, the story is more complicated (black has the strong reply ...Nc5, but for me it doesn't matter for the moment).

When I searched good ideas for white, I saw the two weak squares, but my thoughts were : "Okay, there are two weak squares, but 1/ Nd7 can take at b6, it is well put at d7 to do that 2/ for e6, Nf6 can move to e8, and moreover Bg7 can move to f8. So the two squares are not useful for me".

As a chess player, could you give me some reasons for not rejecting the positioning of knights or bishops on weak squares? Should I always try to put them on such squares, hoping the exchanges will end with a better position for me?
It is true, that sometimes occupying weak squares with your pieces will just lead to exchanges and no advantage whatsoever. But there is a well known dictum in chess, that "the threat is stronger than the execution". 
This means that just threatening to occupy a weak square can severely restrict your opponent's possibilities. Just imagine you brought a knight to c4 and your opponent was set on the idea to either play Nd7xb6 or Nf6-e8 if you dropped your piece into b6 or d6. This would leave him unable to move either knight until you executed your threat. Which in turn would hamper his development, because his bishop cannot move, which in turn means his rooks cannot be connected. And of course you wouldn't oblige him and execute your threat, you would just keep it hanging over his head. 

On the other hand, in this specific position, taking the knight on b6 wouldn't solve blacks problems. After you retake with the bishop you actually control the square d8. This is very relevant, because it means that he cannot oppose you with his rooks on the d-line. So, bringing a knight to b6, retaking with the bishop and then taking control of the d-line by doubling your rooks, seems like a valid plan to use the weakness of b6.

Of course these are purely strategic musings, it is always the question whether a plan can be executed tactically.