It just makes en easier image for your brain.
For example:
If you have 3.27 pizzas and you want to buy it for 4.57 people (we're being hypothetical here, so please don't get shocked). If we try it out in our brain or paper, we could mess it up placing the decimals in the answer by accident, because we're humans. So we estimate it by rounding it, like 3 pizzas and 5 people, then that makes the life easier here - just 3 times 5 and 15 pizzas. So when we try multiply the actual numbers, we could place it around the number which it's close to 15.
(actual answer: 14.9439 pizzas)
Sorry for my bad english.