Brushiing helps remove the stuff that comes out of the plaster finish as it continues to cure. Brushing is important in ANY pool, not just new plaster, and is somethng that I bet NONE of us really do often enough. It helps keep your pool clean and mixes the water in case there are any 'dead spots' where the circulation is not as good. Even though it's a lot of work don't slack on the brushing with new plaster! It will ensure that your plater finish looks great once it's cured.
Brushing is very important for a plaster pool if you want a smooth surface. If you don't get the dust off the surface, it can harden and you will have a sandpaper finish. I brushed my 3 times a day and now it is very smooth.