I find it a bit harsh how many people say "They're adults, they'll be gone soon anyway, so they can just be given something labelled as 'spare' in the meantime".
Hotels are great, but they're still designed to be functional and are not your own home. Yes, you've had a relaxing, enjoyable night; but now it's somebody else's room and you have to pack up and leave promptly. Being able to 'find a room/bed' for a family member who still considers it their own home (at least for a part of their time) or naming the room where they sleep and store a lot of their own stuff as 'spare' is not going to help them feel important.
Even more so if you're either considered 'lucky' because your home is split between two properties; or worse, when both parents treat the fact that you aren't there full-time as justification for you to be an afterthought rather than a focus; talking of our own room 'being wasted' when you're at your other home. University is a bit different, as it's your first foray into the adult world and slowly becoming independent in time; but when it comes to children whose parents are no longer together, it isn't actually the children's choice to no longer have one full-time room in one full-time home.
I think it's easy to overlook the strong emotional needs and feelings of security that young adults rely on. Of course their needs are very different from those of young children, but that doesn't make them any less important. Many will still feel vulnerable and learning their place in the world.
Others may well disagree, but I always think that children/young adults leaving home should be entirely led by them, to make the decision for themselves, in their own good time, with gentle support available for whatever they decide. Naturally, that doesn't mean they're just waited on hand and foot and not expected to play their part in chores, consideration, respect and keeping the household functioning and happy for everybody.
Obviously, if they're still there at 42 with no signs of ever leaving, you might want to 'have a little chat'; but it's horrible for any young adult to feel that their clock in the family is ticking and that they're starting to be seen as a liability who is 'room blocking' and causing problems by not just hurrying up and leaving.