The reason why perimeter and area is important to designers is that when a designer is creating a blueprint they must know the perimeter and area. If they don't then the building/structure will be lopsided. people who build things should know the P and A but you should also know how to calculate the P and A too. It's important to know them and to calculate them. If a carpenter doesn't know the P and A of the area he's working in it could mean trouble.