Gonna heavily disagree with this one... This is an attitude that turns us into machines with expected outputs rather than people. We humans spend a lot of time limiting ourselves by placing labels on ourselves and limiting our perceptions by labeling others.
Discard the labels. Do not try to fit a role. You might program computers, but you are not a programmer. You fly planes, but you are not an aviator. You play a guitar, but you are not a musician. You teach, but you are not a teacher. All these terms have extra connotations we have all built up.
Your footprint on the world is a culmination of your actions, so in a sense, your actions mark you in the perceptions of others. However, as we humans are so adaptable, we can discard the notion of who we think we are and start afresh, always pick new directions. We do not have to be what we have been, and we do not have to cater to what other people expect of us.
Just because you get a college degree in X and work 30 years in X does not mean you cannot ever do Y; that is a self-imposed limitation reinforced by the X label.
Purpose of life? To live. Nice and fundamental. Meaningless enough to show you that you can do about whatever you want if you have the interest and make the effort. Limitless enough to give hope that if, in your 20s, you feel you are not doing what you want, you can try something else.
No, you are not what you do. You are you. What you do is what you have chosen to do at that time. It is okay to change, okay to do the unexpected, okay to break out of your comfort zone and whatever comfort zones and roles that TV and society have tried to impose upon you.