When your body is supported, there is even more space for your mind to then focus. You're not constantly shifting, you're not constantly adjusting, or you're not thinking about how uncomfortable you are but you're just working, learning, or relaxing without any distraction.
Poor posture is a cause of fatigue that builds up slowly. Often, we do not realize that this is what occurs. Removing that pressure allows one to commonly feel more alert with productive all day. Even simple tasks feel easier without discomfort there to distract you.
Proper chairs for ergonomics provide other benefits. It changes also how your whole day goes. It helps your mind stay in focus, your energy last for longer, and your body feel supported.