What is the purpose of phase matching when drawing a MO diagram?

During the lecture, we were learning how to draw the MO diagram for $\mathrm{BF\_3}$. 
I understand that only orbitals with the same symmetry labels can interact and form molecular orbitals. My professor always elaborates a lot on "phase matching," and he would draw the orbitals out.


However, I do not understand how these orbital drawings are supposed to assist me when drawing the MO diagram. How would it affect my process for drawing the MO diagram?


[![enter image description here](https://i.stack.imgur.com/71eUI.jpg)](https://i.stack.imgur.com/71eUI.jpg)


