Observe the motions of the planets in the night sky without ever leaving your parlour.
Grand orreries were educational tools and status symbols, affordable only to the most wealthy.
An orrery is a mechanical model of the Solar System - a kind of early version of a planetarium. A grand orrery is one that includes the 'outer planets' beyond the asteroid belt.
Although mechanically driven, the orrery can be speeded up by inserting a handle in a socket and rotating it. Around the outside of the orrery there is a brass ecliptic scale which show the degrees of the zodiac, the corresponding dates for the Sun's position in the zodiac and the celestial longitude. It represents the relative motions of the six planets (to Saturn) and their known moons around the Sun.
Earth (a replacement model) has its single moon, Jupiter has four and Saturn has a ring and seven moons, although these satellites are not mechanically driven. It was possible for the user to set the instrument to a particular date and time so that it would set the planets to the correct position for that moment.