The reduction of external stimuli helps our brain to calm down
The right meditation posture also serves to some extent for sensory deprivation [stimulus reduction] in order to calm down physical processes.
We sit as motionless as possible on the meditation cushion and avoid all unnecessary movements such as scratching or fidgeting. The immobility of the body and the reduction of external stimuli helps our brain to calm down. Better sitting. Less distraction.
Ultimately, the purpose of the stable, calm meditation posture is to calm the body and mind – seclusion free of distraction. This in turn is supported by the concentration process of the focused mind.
A focused mind is free from distractions.
You can also call meditation also a body-mind-synchronization-training. It is a mind-altering embodied being process.