Answer:
A bed-liner protects your bed from getting stained and that worn look. A truck bed-liner does about the same, except it protects the metal from being scratched.
Mattress covers that encase the entire mattress on top, sides, and bottom (under the bed) give some protection against dust mites. However, bed bugs have been shown to be able to crawl out of the encased material, usually near the zipper area.