Using something flat like cardboard or newspaper as the bottom layer ensures the already-growing grass or whatever can't find a way back to the light and air, and renders most seeds already in the ground un-sproutable, except those which happen to become exposed when you're digging a little hole later to add a plant.
If you put a smother down now, it should be well decomposed by spring. This decomposition process also greatly improves the soil by adding organic matter and, as the worms and smaller members of the decomposition crew do their thing, the nutrients are processed into a form and deposited at a level in the soil that roots can access and deliver to the plant(s.) This also moderates moisture levels in times of drought/too much rain, and keeps soil from getting unnecessarily hot/cold, and from being eroded by weather.
Covered with leaves, compost &/or mulch, it just looks like a flower bed with nothing in it yet.
