This is almost a common sense answer. Firstly the ground must be fertile to produce grass. Rabbits come and eat the grass. When rabbits are numerous or have vulnerable young, the hawk will be hovering overhead waiting his opportunity to grab one.
The fecal matter from rabbits will nourish the soil to some extent and help the production of grass. In the food chain, each element is important, which is why less popular animal forms should not be eradicated.