I think we’re using “purpose” differently. I’m talking more in the existential sense. Perhaps there is some overlap in the way you’ve mentioned bodily organs. However, I think most would agree that the “purpose” of the liver is not the same as the 45 year old at midlife crisis asking “Why am I here?”
As an existentialist (even a theistic one), that triggers a whole different set of objections, to say that our purpose in the existential sense is one which we discover or decide for ourselves. And as a theist existentialist in particular I would say that this because rather than being created as tools for an end, we are children created as an end in ourselves. So you may have to find a different qualifier like “created purpose,” in which that very assertion that we are created as children would be the answer to that. I certainly don’t buy into the needy God idea that we are created to love, worship, and glorify God. Instead I would insist that these are all things we do for our own benefit, because they help combat the self-destructive habits of sin or because they help open us up to all that God has to give us. Instead the reason we are created is because being without need, God is naturally motivated to give of His abundance and so God created us for a relationship of eternal parent with eternal child, where there is no end to what He has to give and no end to what we can receive from Him.
That’s right. This is the meaning of the existentialist maxim “existence before essence.” First we exist and then from our experience of existence we find out who we are and the meaning of our lives. This maxim was first stated by Sartre who is not a theist, so I explained a fully theistic version, that instead of being created for an end like a tool, we are created as an end in itself like child.
So the problem with the phrase “existential purpose” is that this is going to embroil you in the philosophical writings of existentialism, so I suggested an alternative phrase “created purpose,” as being closer to what you mean.