[detaro]

It's an old unit, so there wasn't a global standard yet, and it's exact value wasn't known for a long time, but it was still useful for ratios. E.g. if you observe the orbit time of another planet, you can tell its distance from the sun relative to the distance of Earth to Sun (1 astronomical unit) relatively accurately, even if you can't measure what it is in meters very well.

[Taek]

For my purposes 10 AU has a lot more meaning than 1 billion km. 1 AU has a lot more context

[dylan604]

It's one of those things that I still tilt my head slightly at when thinking about how the human brain struggles with really large numbers. Its just an odd thing.

[II2II]

In a case like this, I suspect it has more to do with visualizing the solar system rather than how large something is. When we speak in AU, we can create a mental model with the Sun, Earth, and other object since everything is normalized to the size of Earth's orbit. When we speak in kilometers, a bit of arithmetic has to be done before creating the model. Regardless of the units though, we aren't directly visualizing the distances since they will be outside the scope of human experience until we actively start traveling the solar system.

Edit: for clarity.

[Pelam]

I have more trouble with parsec. I have a rough idea of how big our galaxy is in light years and some idea about nearby stars. Age of the universe helps anchor things in billions of light years. Then suddenly something is measured in parsecs. Probably a similar thing for the experts.