Breaking complex or difficult shapes into smaller ones or into shapes that are more easy to get the area of or the perimeter makes it easier if you don’t understand or know the formula for the shape. For example, if you are working with an irregular shape that you need to solve. For example, in a worksheet that we had been given, area on the Geoboard, we had been given numerous irregular shapes that we need to get the area of, so instead of trying to get the area of the entire irregular shapes and taking a lot of time, I had divided the irregular shape into triangles, trapezoids, square, etc, so that it would be way easier to get the area of each shape. And if you don’t know how to get the area of these shapes/the formula, you can divide them into even smaller pieces, and then add the smaller pieces together to form a shape that you know how to get the formula of (for example, a square, rectangle, triangle) or so that you can actually count the area (grid paper, which you can count the units the shape occupies).
For example, in this irregular shape, I had moved a triangle to the other piece of it to make a square, next, I will split the parallelogram into triangles, then make them into a square as well. After that Since there is a total of 4 squares, the area is 4 Units squared.