it is used because they can trace back how old a layer of strata is and when they find how old that layer is than if there is a fossil within that layer than the will know how old that fossil was
Superposition is the principle that states that, in undisturbed strata, newer layers will be deposited over older layers; thus, in a core sample, those samples of earth and rock nearest the surface...
Because if you know how old the fossil is, and you find the fossil inside a rock, you know that rock has a maxium age corresponding with the age of the fossil.