Use the adjective functional to describe something that is made to do a specific job, such as the functional alarm clock feature on a digital stopwatch.
The word functional comes from the Latin word functionem, meaning "performance, execution." It can also describe whether something is working properly. This is what shoppers at a garage sale want to be sure of before they leave with an old television — they plug it in to make sure it's functional. In this case the opposite of functional is useless.