The comment bellow is from reddit so take it with a grain of salt:
tl;dr: Being in the same position for a long time is bad for you. Being in an uncomfortable position forces you to shift around constantly, effectively keeping you from staying in the same position for too long.
ts;wm: Physiotherapists, gymnasts and other people who move a lot with usually say something like "The best position is the next one". Move around a lot, don't get stuck in the same position.
A 2 year old is more flexible than 90% of adults. Because that's how we're born. The only reason we lose it, is because we don't use it.
What's so bad about losing it? Back pains, knee pains, other joint pains, limited mobility, etc. Your body will create faulty movement patters because muscles are underdeveloped or overactive due to being in the same position all the time.
This just degrades over time. You can go for decades without noticing real issues (because you don't need the range of motion in your life), until you notice that walking up stairs starts to hurt your knees, and standing up for more than 2 hours at a time starts hurting your back.
I don't like the whole "Make sure you're sitting properly" ergonomics thing. Sitting int he correct position will make it take longer before you start experiencing issues, but it's not a solution. You're just degrading your body in a more controlled way, rather than not being in the same position for 8 hours a day.
All we do in modern life is sitting and transitioning to other places to sit. Wake up, sit at the brakfast table, walk to the car, sit while driving. Walk to our desk, sit at it. Walk to the toilet, sit on it. Go to lunch, sit at the table. Go home, sit on the couch watching tv.
It's not hard to understand why nobody can touch their toes anymore and develop hernias. Humans evolved when running, farming, gathering, and just general going around was the norm. The Wall-e future has pretty much already happened, apart from small walks of maximum 20 meters a few times a day.