I’m referring to Fermi estimations. Yes, NASA uses 15 decimal points for pi, but astronomers aren’t always making super precise calculations. As I mentioned in my previous comment, it’s used for estimating orders of magnitude. It’s helpful when precise calculations are complex, because any error along the way could be obscured or glossed over. A decent fermi estimation will help you identify when your precise calculation is wrong. This estimation can often be done quickly with very little actual data, because you’re only looking at orders of magnitude and rough numbers.

Let’s say you’re trying to calculate something complex. Your Fermi estimate takes like two minutes, and says that the answer is probably in the ballpark of ten million. Your precise answer takes an hour, and comes out to be nearly a billion instead. You can look at your fermi estimate for a minute or two to see if you missed a zero or two somewhere. And if you didn’t, then you need to scrutinize your complex calculation because you know you made an error somewhere.