Why do heliostats need to move when the furnace stays still?

A heliostat (from helios, the Greek word for sun, and stat, as in stationary) is a device that includes a mirror, usually a plane mirror, which turns so as to keep reflecting sunlight toward a predetermined target, compensating for the sun's apparent motions in the sky.
en.wikipedia.org/wiki/Heliostat

If you didn't have a heliostat you would have to move the furnace.