SMPSes are also used for DC/DC conversion (up or down). Especially with solar panels, they are mixed current/voltage-mode devices, so you want to load them at a

SMPSes are also used for DC/DC conversion (up or down). Especially with solar panels, they are mixed current/voltage-mode devices, so you want to load them at a specific voltage (which depends on the incident light) to maximize the power draw. (This is what a power optimizer does in a solar panel installation.)