Would like to know how a glossy screen is far more useful, and under what conditions?
Your paraphrase sort of takes my comments out of context. It isn't far more useful in general, and in fact most of the time it is pretty much irrelevant which type of screen it is. But in fact it is far more useful in a lot of situations, and in fact the brighter the ambient light the more helpful the glossy screen is.
I find the screen usable under any nearly any lighting conditions, including outdoors in daylight. Reflections are rarely a problem - you normally can't even see them unless the screen is off. Because reflections are specular, slight screen adjustments can usually resolve them, but to be honest I can't remember the last time I adjusted the screen to avoid a reflection. A matte screen on the other hand has the same reflections, but it's diffused, killing contrast and making it difficult to see the image. Sure you can't see a reflection, but you can't see the image either. The brighter the ambient light, the tougher it gets to see a matte screen.
An example, sitting in the front seat of a car riding to california in day time (which I do several times a year), old MacBook Pro required full screen brightness and still challenging to see and read info. Glossy screen no problem with screen turned down to 75% to conserve battery.
Obviously these aren't ideal conditions for working on photos, but that's not the main use of my MacBook anyway.
I didn't think I'd like it ... but now I prefer it. Been using it since Apple first introduced it a couple of versions ago, so this is the third MacBook Pro I've had with the glossy screen.