I can teach technique, but the expression of a painting comes from within you and is unique to you.
Glazes are extremely useful in altering a picture whilst still keeping it looking fresh. It makes colours appear luminous and creates beautiful nuances not possible by mixing colour in the palette or on the paper, wet-in-wet.
If you did the exercise Using Tone To Create Aerial Perspective, you were glazing. You will have noticed that non-staining colours lift and that tone is easily controlled by utilizing the glazing technique.
There are other purposes for using a glaze-: