I am not sure, but I think it may be for the fry to hide in. I think you mean the plant called hairgrass? I have read that it provides good, and a lot of, hiding places for the tiny fry. But you may want to wait for some other replies to your post as I am not 100% sure.
Breeding grass is used forlivebearer fry to hide in from the parents right after birth, for egglayer fry to eat little microscopic critters from after hatching and for egg scatters to scatter their eggs in Guppy Grass(Najas Guadalupensis) or Java Moss (Vesicularia Dubyana)
These are th most common ones used. Hope that helps.