In the section on Basic Knight Strategy, we saw that it is advantageous to place knights in the center of the board where they cannot be attacked by pawns. Here, too. You can see that it would be useful to place a knight, or another piece for that matter, in the "hole" at d5.
By placing the knight in d5, or even by using the knight to control d5, we prevent black from pushing the pawn from d6 to d5. As you can see in the diagram, white controls the key d5-square with both the knight and the pawn on e4. If black pushes the d-pawn, white will win it. The pawn on d6 has therefore been fixed.