RV travelers and cabin guests often deal with limited storage, changing temperatures, and bedding that is functional but not always ideal. A sleeping bag liner fits that lifestyle well because it gives you a personal sleep layer without taking up much room in a cupboard, duffel, or under-seat bin.
It is also easier to wash between stops than heavier blankets or sleeping bags. For people who travel often but still want a more comfortable and consistent sleep setup, a liner is one of the simplest items to keep packed and ready.