In the image above, imagine a knight on H3, which covers g1, g5. Neither a bishop, a pawn or rook can help that way.

After one move, it can go to f4 where it can add on as a second attacker to the queen on 4 squares. After another move it can go to d3, where it can again cover squares the queen cant (e2,e5).

So I guess its both because it has a lot of alignment in doubling as an attack, and covering nearby squares the queen cannot.

There was a big discussion of this topic on chess stackexchange[1]. It used to be just universally believed to be true but some modern analysts challenge the idea.

[1] https://chess.stackexchange.com/questions/27815/why-does-a-k...