Here is the explanation to this question. This is related to our center of gravity or also known as centroid. When we are sitting upright, our center of gravity is not over our feet. By leaning forward first before we rise, this is to transfer or shift our center of gravity vertically over our feet, which makes us easier to rise up without falling. On the other hand, we can still rise up without leaning forward, given that we can use the chair as our leverage. Hope this answer helps.