The practice of moving from position to position while coding encourages you to ask your body what it feels like at any particular moment. You can then create a password that will support whatever the current feeling is. For instance, if you feel energized, then you can choose to stand. If you're engaged, then you can perch. Checking in with your body lets you take better care of it.
You can just turn and walk away from your workstation, to think, to get a drink, to look at distant objects, etc. If you're sitting, a need to get out of a chair can be deciding factor whether to continue being where you are or get a stretch.
If you are sitting, first thing which comes to mind is to lean back and relax, which will make your back perfectly round after several years of doing so.
Seriously, after 8+ hours of sitting all of my back becomes constantly covered in sweat. My personal experience, though. To alleviate this you have to buy expensive chairs, which is a hindrance.
Prolonged standing can lead to increased fatigue, leg cramps and backaches, all of which can negatively impact job performance and cause significant discomfort.
You can use the chair with your shins on the shin pad, as intended, or use the shin pad as a foot rest for one or both feet. Being able to switch it up can be very helpful if you get restless sitting still very easily, but don't have the option to get up and move around.
Kneeling chairs position you with an open hip angle. This eases your hips forward to encourage an upright posture, aligning your back, shoulders and neck.
While some studies have shown these style chairs to be good for maintaining posture, other studies have suggested they may be no better and possibly worse than a well-designed traditional chair. Prolonged usage of the shin pad with both knees (sitting as intended) may lead to decreased circulation to the lower legs.
The act of standing means you need to support yourself against the force of gravity and the treadmill forces you to walk. Neither of these are very enjoyable.
If you stay leaving on a chair, you'll lack back muscle and you'll likely have back pain, leading to worse posture, leading to even less muscle, etc. A yoga ball forces you to muscle your back.
Along with the general back muscling, it will develop small "stabilizer" muscles that frequently don't get enough activation. This can actually help make you less prone to certain types of injuries.