Without using the pronominal/reflexive  “Je me regarde les pieds”, you don’t necessarily know whose feet ‘Je regarde les pieds” means - it is not totally unheard of to look at other’s feet, so unless the context makes it clear,  “les pieds” does not necessarily refer to your own feet, whereas “mes pieds” clarifies the situation. As well as the lesson you referred to:  Using le, la, les with body parts and clothing (definite articles)

 there are situations where possessives are used, covered in link : Special cases when you use mon/ma/mes/etc with parts of the body (French Possessive Adjectives)