A sleeping bag liner earns its place in a travel bag because it solves several problems with one lightweight item. It gives you a cleaner layer between your skin and unfamiliar bedding, adds comfort in hostels or guest rooms, and packs smaller than carrying extra sheets or a full blanket.
For travelers who move between hotels, train berths, cabins, and campsites, a sleeping bag liner is one of the easiest ways to stay prepared without adding bulk. It is quick to wash, easy to dry, and practical whenever the sleep setup is less predictable than expected.