Linear feet and square feet measure different things. For example if you have a 2800 square foot house then this is a measure of how much land the house covers. If you plan to put a piece of trim all the way around the outside of your house then you need to know the length of trim required in feet, or sometimes we say linear feet.
There is no direct relationship between the area, measured in square feet, and the perimeter, measured in linear feet. Your neighbour might also have a 2800 square foot house but the shape can be quite different and hence the perimeter of his house might be quite different than the perimeter of yours.