A binder is a material, usually sprayed on the soil to bind the layer of soil and prevent water and wind erosion. Also helps hold seed while it germinates. Really useful when seeding hillsides or slopes.

I'm not sure if that's the same stuff you have on your hands or not. Sounds like you spread and water in this binder vs mixing in a spray tank and spraying on.

I've never seen a product under that name sold separately, but I'd assume it to be some sort of cellulose material. This is a common ingredient in spray applications of grasses for large areas (hydroseeding), and as grass guy indicates, assists in providing an even seed distribution, helps to prevent the blowing away of the seed and holds moisture and any added nutrients until it germinates. A similar product is incorporated into those lawn "patch kits" you can purchase. Totally biodegradable.

Won't hurt, could help :-)