The most common role of sleeper ships in fiction is for interstellar travel, usually at slower-than-light speed. Travel times for such journeys could reach into the hundreds or thousands of years, making some form of life extension such as suspended animation necessary for the original crew to live to see their destination. Suspended animation is also required on ships which cannot be used as generation ships, for whatever reason.
Suspended animation can also be useful to reduce the consumption of life support resources by crewmembers who are not needed during the trip, and for this reason sleeper ships sometimes also make an appearance in the context of purely interplanetary travel. The famous movie and book 2001: A Space Odyssey, for example, features a ship travelling to Jupiter (Saturn, in the book) with most of its crew in suspended animation and only a handful remaining awake to operate the spacecraft.
In contrast, see Generation ship.