Using a sleeping bag liner inside your main sleeping bag helps reduce the amount of body oils, dirt, and everyday wear that reach the insulation and inner fabric. That matters on multi-night trips, especially when washing the full sleeping bag is inconvenient or not practical between outings.
A liner can also make cleanup easier after travel because it is smaller, lighter, and faster to wash than the full sleep system. For many campers, that convenience is reason enough to carry one, even before considering the added comfort against the skin.