I'm not Wayne, but the 1 degree disc is for grinding pieces that are longer than the half of the disc you're grinding on. As the disc rotates clockwise, for instance, you'll have the grit on the left side of the disc moving upwards, while the right side is moving downwards. This make things a little more tricky to hold if your disc is pulling your piece is two different directions at the same time.
When you have a 1 degree bevel on the disc, as long as the piece is relatively flat, you will only be contacting one side of the disc at a time, even if it moves past the center where the direction of the wheel changes.
I have both the flat and 1 degree in my shop. I use the 1 degree 99% of the time. Mostly because it is 4 steps closer to my bench.
The 1 degree does put a very slight hollow on your material. Something like a tenth of a thou or two over 2". You really can't hand hold it more accurately on a flat so it's a non-issue. You can use this hollow "feature" to get rid of stubborn spots. by scrubbing your material across the disc. If I had to chose between one or the other, I would chose the 1 degree.