Hallways are used to connect rooms to other rooms or to one of the exits. Hallways contribute nothing to the efficiency of your dungeon (other than as a place to lay down traps) and should be avoided as they count against your max tiles. Keep in mind that you don't need to have all of a specific room type contiguous, so you can convert any extra hallways into storage room tiles for extra storage, farm tiles for extra food, or mine tiles for increased mining rates. If the tiles are near the entrances, however, it's not recommended that you use tiles that can be looted, unless you don't intend to defend from raids. For these areas, perhaps the best type of tile is the library, as that will allow you to have more researchers, but they can't be looted from, or guard post which will increase the damage dealt by your creatures.
Tile Cost: Hallway tiles do not cost any Gold to build.
Per Tile Bonus: Hallway tiles do not provide any benefits.