When my friends see me sit on the floor instead of couch, they often ask me why I sit on the floor because it's not comfortable.
First of all, it's much more comfortable for me to sit on the floor than to sit on a chair or couch no matter how fancy or ergonomically designed they are. Besides my personal comfort, there are several reasons why I advocate floor sitting.
It is not so much a surface you sit on, or a position you are in. For me, the biggest difference is in movement variability from floor sitting vs chair sitting. When you look at all possible movements that can take place during transitions from sitting to standing, standing to sitting, and compare floor sitting and chair sitting, you will notice right away that there aren't that many variety in the way you move from chair. Sitting on the floor, on the other hand can provide many more options in a sitting position and movement options from each position.
So, why does movement variability matter? To understand it you will have to put in a context. Here are a few contexts: 1) Flexibility/Mobility 2) Strength 3) Coordination/Balance 4) Health of Joints
1) Flexibility/Mobility: Let's experiment to understand this: Try SLOWLY sit on the floor and SLOWLY get up and notice how much movement this action involves at toes, ankles, knees, hips, spine, ribs, arms. Now, try do this from a chair. I think the difference is very obvious. Imagine how much of difference this could make over months and years. Our bodies adapt to demands placed on, thus those that are used more become flexible and mobile, and those that are not lose flexibility and mobility over time. No wonder why those who grow up in countries where sitting on the floor is common have, in general, good flexibility and mobility even when they are in their 80's.
2) Strength: Do the same experiment as above, and notice any differences in muscles you engage. Moving from the floor and to the floor requires more movements at all joints, meaning it also requires more muscle engagement, particularly in hip muscles, which is the "powerhouse" as it generates the greatest power in our bodies. "Use it or Lose it" principle applies to this.
3) Coordination/Balance: This may not be as obvious as the first two. To illustrate this domain, observe toddlers and/or judo/aikido masters how they move to the floor and from the floor. You will notice gracefulness, softness, smoothness, ease, and elegance, which are some of the characteristics of coordinated, balanced movement. What gives movement such quality is the use of the entire body and coordination among the body parts so all parts are working in harmony. Movements from the floor involve more body parts than from a chair and to make transitional movements easier requires improvements in coordination among all body parts, which is essentially improving your balance.
4) Health of Joints: There's a saying "Motion is Lotion." Joints naturally produce lubricants for themselves, and the production of joint lubricants is stimulated by movement. Thus, as movement decreases, joints produce less lubricants and eventually dry up. Movement is literally essential to our life as movement increases circulation and all joints receive essential oxygen and nutrients through blood. When joints are deprived of movement, they are deprived of nutrients. It's very obvious what will happen, isn't it?
While there are many benefits in floor sitting, I'd like to mention to you that floor sitting may not be appropriate for everyone, especially if you haven't sat down on the floor for many years for some reasons.
In conclusion, from my perspective, the real value of floor sitting is in movement potential it creates rather than a position.