Sleeping on the hard surface, such as the floor or a bed frame with no mattress, is not a question of comfort level, but mostly a question of keeping a good posture and body alignment. Because sleeping on hard surface is generally not comfortable, people would still sleep on hard surface for therapeutic reason, such as correcting their poor posture, or for other spine alignment purposes. The other benefits of sleeping on the floor is the habit that would be obtained out of doing this practice. One study shows that when you regularly sleep on a hard surface such as the floor, it improves your posture and you will have a better sleeping pattern.
Some people might question these benefits of sleeping on the floor. There would be skepticism because our general preference is to sleep on a nice, soft bed. However, sleeping on the floor does not necessarily mean sleeping directly on it. Of course you can put a thin blanket or a cushion between you on the floor. It may not be as soft as an actual bed, but the straightness of the floor as your body lies on it can resort to correcting poor body posture.
As an alternative to sleeping on the hard floor, one can try sleeping on cot beds instead. This is not as hard as the surface of the floor, but also straight and may entail the same impact to body posture. Another alternative is sleeping orthopedic beds. This is a specialized bed designed for people who are generally having discomfort sleeping on regular beds. Orthopedic beds are designed with curvature that aligns perfectly to the curve of the body. A lot of reports say that this is the most comfortable type of bed, but of course it also comes with the price.