Question

How can estimation help me to find the area of a rectangle or square

Answer 1

First, I'm going to categorize the portions of the rectangle. The red portions are grid squares that are completely enclosed by the rectangle, the green portions are the corners and finally the blue regions are the edges. 

Answer 2

How it can help is if you have one side of a rectangle like 4.2342323332 and you had to multiply it by 2.8493844, that would be too hard so you could just make it into 4x2 which equals 8