Degaussing has to to with the metal grid at the front of a tri-color single tube CRT. these grids are made of ferrous materials. So, these devices can need degaussing, to be sure the grid has no residual magnetisim. Magnetism will shift the beam.. (as it tries to go through the proper holes for each color) creating color shifts, or color spots.
The tubes in a Projector have no grid (they are actually called 'electron masks', or 'shadow masks').