These days the Astronomical Unit (AU) is defined exactly (149597870700 m), but historically AU was used to measure relative distances in the solar system becaus

These days the Astronomical Unit (AU) is defined exactly (149597870700 m), but historically AU was used to measure relative distances in the solar system because the distance from the Earth to the Sun was unknown. The transits of Venus in 1761 and 1769 were used to determine the distance to the sun and hence the scale of the solar system in a major scientific endeavor. Several expeditions were shipped around the globe to do simultaneous observations of the beginning and end times of the transit, using the positions of moons of Jupiter as a time reference. The AU is still in modern use because it can easily communicate distances around the solar system to humans. It is less useful in computation and it's a somewhat arbitrary number given that the orbit of the Earth has minor perturbations.