There is an underground..."battle of the root systems"... for nutrients and water in the soil, between weeds and desirable species. Canada thistle (one of our worst noxious weeds) can out-compete smooth brome and Kentucky bluegrass for nutrients and water in the soil, because it's root system extends much deeper into the soil.
However, because of their very extensive root systems, native grasses such as big bluestem, can out-compete Canada thistle for nutrients and water in the soil. Once established and managed properly, the native grasses effectively keep weeds from ever getting established. The ultimate benefit of managing native vegetation on roadsides, whether remnants or re-established, is that pesticide use will decrease, mowing costs will decrease and the wildlife habitat value of roadsides will improve.