Making rough estimates of physical quantities is useful

So that you can see if the answer to a problem makes physical sense.

Explanation:

Examples of this would be when you decided to choose a box that you are going to use to place your rice + Vegetables for lunch.

In order to do this, you won't necessarily count the amount of rices and vegetables and compared it to the box, but you are most likely will make a rough estimates of the physical quantities to choose the box,.