To show how ludicrous an amount of money it is. That sustainable harvesting of it gives an annual amount of money that any reasonable person would consider set for life, and then to show what that looks like, because it’s not a “well if you live within your means you can retire young” amount, it’s an annual amount that people who make it can budget wisely and retire young on. That vast fortune is every year’s return on investment for a billionaire.

To frame it differently, it’s to express just how ludicrous the amount of money you gave for the annual return is, because even that number is so big people don’t really get it.