A sleeping bag liner and a travel sheet solve similar problems, but they are not always used in exactly the same way. A sleeping bag liner is often chosen by campers and travelers who want something that fits well inside a sleeping bag while still working on its own in hostels, cabins, or guest stays.
A travel sheet may be enough if you only want a very basic barrier for indoor travel. If you want more flexibility across camping and general overnight use, a sleeping bag liner is usually the more practical option because it adapts more easily to different sleep setups.