Most products don't last forever. Products designed to have secondary usages after their primary function has lapsed can add value to the product, and may fill a need that would be filled by another purchased product instead.
Example: Adding graduated lines to the side of a salad dressing bottle makes it useful as a measuring device after its initial use.
Watch out: Trying to work unnecessary second uses into a design can often increase the complexity. Design a product's second life such that it requires no extra parts or complexity that wouldn't be needed in the original design.