It can be nice to have a spare room in your home in case you need the extra space. If you're not doing anything with it though, it's probably costing you money.
As The Simple Dollar points out, if you have a room (or rooms!) that aren't being used, you're paying for space without getting any benefit in return:
Many people view their empty room as a “guest room” or something like it to justify the expense, but unless it’s almost constantly in use, you’re losing significant money on that room — money which, sometimes, you really can’t afford to be losing.
If you have one or more guest rooms, consider subletting them. Or, if you're the adventurous type, get paid to host space on Airbnb. If you have a storage unit, get rid of it and move your stuff into the extra room. It may be cluttered when you have guests over once or twice a year, but you'll save a lot of money in the long run.
The Expensive Extra Room [The Simple Dollar]