A fixed unit of length
One au is about 149.6 million kilometres. Its exact metre conversion is a definition, rather than a new value obtained whenever the Sun is observed. Distances reported in au therefore share one fixed scale.
Earth’s actual solar distance changes around its orbit. Saying that Earth is roughly one au from the Sun describes the scale, not a constant instantaneous separation.
References: [1]
Comparing Solar System distances
Planetary and spacecraft distances can require unwieldy kilometre values. Using au makes comparison with the Earth–Sun scale immediate: two au is about 299.2 million kilometres, not two years of travel.
A distance still needs endpoints. A body’s heliocentric distance normally differs from its distance from Earth, and both can vary with time. A shared unit does not specify that geometry.
Relations to parsecs and light travel
A parsec is defined as exactly 648,000/π au, linking this Solar System scale to stellar-distance units. Larger stellar distances are commonly expressed in parsecs or light-years.
Dividing a distance by vacuum light speed gives its light-travel time. One au corresponds to roughly 499 seconds, or 8 minutes 19 seconds. This conversion does not turn au into a time unit.