A spare bed for visitors takes up a lot of space. A trundle bed that slides under a bottom bunk bed is a good way to have an extra bed for sleepovers without taking up more floor space when it's not in use.
Bunks beds with a trundle bed underneath work well for siblings that share a room and sometimes have a friend to stay. Our boys share a room and have similar age cousins so we need to have plenty of beds to fit everyone in.
Bunk beds plus a trundle bed also work very well in vacation rental properties and hotels. You can fit three people into quite a small room because the trundle bed can be pushed underneath when not in use. It also works well for guests who have two small children where the oldest is not yet big enough for a top bunk.