Somebody else already said it, but that's what the title is.

Longform: a lot of calculations that happen in astro deal with distances so large so large that only order of magnitude changes actually meaningfully affect the end result. To connect to a more common topic, here's a joke.

"Whats the difference between a million dollars and a billion dollars?"

"About a billion dollars"

This joke works for the same reason; 1 billion is so many orders of magnitude larger than 1 million that (1,000,000,000 - 1,000,000 = 1,000,000,000) is only incorrect by ~0.1%, even though substituting 0 for 1 million in that equation seems ridiculous on the face of it. Substituting 1 for pi has similarly minimal errors (tbh it usually matters waaaaaaaaay less than .1% error) in a lot of astro math

Astronomy often has pretty high error bars on their measurements (distance, size of stuff, etc).

In astronomy, the important part of the number is often just how big it is (that is, the exponent). Multiplying by pi doesn't change much in that.

The explanation is in the title.