For renewable power: the voltage coming off of a solar or small-scale hydro system varies so it ends up being the most practical to convert that power to AC the

For renewable power: the voltage coming off of a solar or small-scale hydro system varies so it ends up being the most practical to convert that power to AC then back to the EXACT voltage DC needed for charging a battery bank (or just skipping the "back to DC" step and adding that power to a house's power source.