Since your son is in elementary school, the simpliest answer is: Rounding up or down helps make is easier & quicker for people to understand the number. By rounding up or down it lets people know *about* what the number is & it helps when your not exactly sure the number is accurate.
Example: I have *about* 6 cups left of milk. OR I have 5.87 cups left of milk.
Which sentence is easier & quicker for you to understand?
If you measured the milk yourself, would you rather be graded on pouring *about* 6 cups or 5.87 cups. If you answered *about* 6 cups, you would have the same answer as most scientist or reporters because they wouldn't want to get in trouble either if some of their measuring was not exact!