"The room appears to be empty."
As a player, do those words drive fear into your heart? As a GM, do you find yourself gritting your teeth and wondering why the author of a module would have included another empty room?
Well, take heart. There are good reasons to include empty spaces in an adventure design. A few of those reasons are listed below.
1. The Dread Verisimilitude: Yes, an adventure location seems far more "real" when every space is not packed to the gills with monsters and treasure. One of the main criticisms of the dungeon crawl is that so many creatures live in such close proximity without murdering each other. A really simple solution to this problem, and one that existed when the hobby began, is to include empty spaces.
2. Player and Monster Tactics: Knowledge of the layout of empty spaces allows players to lure monsters into an ambush, and vice versa. Being able to pass through empty spaces may also mean an ability to bypass certain encounters, which may mean the difference between success and a TPK.
3. Somewhere to Rest: Those unfrequented areas of the dungeon make ideal spots for battered PCs to retreat to. Which leads to....
4. Change My Dear, and Not a Moment Too Soon: If areas are empty as the PCs pass through them repeatedly, they can be caught off-guard by unexpected inhabitants. These might be wandering encounters, they might be ambushes (see #2, above), and they might be battered monsters looking for somewhere to rest (ala #3). In this last case, the monsters may not be so eager to leap into battle, and the PCs may have the rare chance to exchange words with a manticore (or whathaveyou).
5. Disguise: Rooms that are actually empty disguise rooms that appear to be empty, but which actually contain hidden traps, treasures, or monsters. If there is something in every room, then the supposedly "empty" room in which something is hidden sticks out like a sore thumb. This encourages "pixel bashing", where having many "empty" rooms actually be empty discourages the same because it is not rewarded.
It should be remembered that "empty" in this case need not mean boring. The "empty" room can have interesting features (aka "dungeon dressing") that point toward a larger backstory for the adventure location. Such areas can contain clues to the nature of the dungeon as a whole - an ancient kitchen indicates that there should be store rooms nearby, and a dining area. Perhaps there is also a way to the surface close at hand, with which the pantries were stocked!
It is not only deadly monsters and traps that deserve a "footprint". The good judge considers how to pass context on to his players at every opportunity.