You probably meet this guy more often in your kitchen than in the bathroom, but yes, onion extract is also used as a skincare ingredient. Its major - but also doubted - skin benefit seems to be that it can improve the cosmetic appearance of postsurgical scars (there is also a scar-fading gel called Mederma, that has hardly anything else in it but onion extract).
According to manufacturer info onion is not only a scar healer, but it has also antibacterial and antifungal activity, it can help to reduce acne and stimulate hair growth.