The snow of winter is important for vegetative landscapes in cold climates, including prairies. It has the effect of insulating the ground, keeping the heat in and cold out. Likewise, it also serves to keep the moisture in the ground. Even though the turf is frozen, if exposed, evaporation happens and the soils dry. And of course the newly deposited seeds from the previous year need moisture to geminate. Moreover, certain native wildflower seeds need to "stratify." Most milkweed species, for example, need a cold/moist stratification to encourage spring germination. The thawing/freezing of late winter/early spring has the effect of bringing them out of dormancy.