Sleeping spots are used to designate a particular patch of ground for your pawns to sleep on. Although technically furniture, they require no materials to build, and take no time for construction or deconstruction.
Sleeping spots provide a bit more comfort than simply sleeping on the ground. They are inferior to beds and bedrolls, and receive no benefit from end tables or dressers. Sleeping spots should not be used for any long-term purpose. Instead, they are for controlling where your pawns without proper housing sleep; left to their own devices, pawns without sleeping arrangements will simply pass out the moment they finish their last task.
Sleeping spots are also very useful for emergency healthcare. Hauling a dying pawn all the way back to your hospital might not be an option in extreme cases. Instead, you can simply build a sleeping spot next to where the patient is lying and assign it to medical use. Once the patient is treated, deconstruct the sleeping spot and let the patient recover in an actual bed. This technique does increase the odds of infection, but that's still preferable to death.