We use catalytic material to control the direction of growth of atoms (ion) on the substrate so as we can get nanotube or nanowire .....
And help us to precipitate some of the films that are difficult to precipitate on the normal substrate because of mismatching between them and the lack of adhesion
It helps in changing the growth habit of the crystal leading to anisotropic growth in specific directions. In other word it deactivates the growth of some crystal faces and activates other faces by adsorbing on specific faces.