Chess has a democratic tendency that tends to make all pieces become more nearly equal in value; I call it the Levelling Effect.
In its simplest terms, when a Queen attacks a defended Knight, the Knight is not afraid; but when a Knight attacks a defended Queen, the Queen is afraid and moves away; and so in a sense the Knight is stronger than the Queen because it is weaker than the Queen.
Similarly, when a piece occupies a strong position, it is difficult to chase it away unless you can attack it with a piece at least as weak; this is the same principle that makes it bad to have a hole in your Pawn formation.
In My System, Nimzovich tells us that the way to profit from a material advantage is to advance your pieces into better and better positions, and the opponent must either trade (which helps you), or allow your pieces to dominate the board. This is how a weak piece can become strong.
Another general principle of Chess is that the weakest pieces should be developed first. Lasker said "Knights before Bishops", Nimzovich spoke of the dangers of the Pawnless advance, and Philidor espoused the advance of the pieces behind a mass of Pawns: these are all specific statements of this general principle.
According to this principle, the Knights have a special role as the shock troops of Chess: they develop quickly and make first contact with the enemy.
In this context, you can see that if your weakest piece is somewhat stronger than your opponent's weakest piece, you will have a hard time avoiding an unfavorable exchange of pieces when the pieces first come together; or if your weakest piece is slow to develop, you have a disadvantage for the same reason.