Hi Sean
Bulbed projectors tend to have a raised black level, due to light leakage past/from the panel(s).
This can give a glow to blacks in dark scenes, and can even wash out the picture slightly.
One thing bulbed projectors normally have in spades is brightness, and some can be spared to get a better black level.
Using a grey screen is a way of lowering the overall light output from the projector.
Black appear blacker, and colours can seem more saturated.
But whites will also appear duller, so you have to try and match your projector and you own preferences.
Some people use neutral density filters in front of the projection lens to achieve the same thing.
But one advantage the grey screen offers is that also helps to reject reflected/ambient light in the room.
Thankfully, the need for grey screens is coming to an end.
The new generation 3000-1 + (contrast) projectors seem fine on normal 1.0-1.3 gain white screens.
Also, new designs using variable iris's can automatically reduce the light output from the projector during dark scenes.