Patanjali defined asana tersely: sthira sukham asanam — steady, comfortable seat. The classical function is to prepare the body to sit still for meditation; the postural complexity of modern yoga developed mostly in the 20th century.
This is not to devalue modern postural yoga, which has become a serious practice in its own right and a doorway for millions into older traditions. But the earlier framework places asana as one limb of eight, not the whole.