I like to think that all forms of existence (living & otherwise) are a direct expression of a process. Call it "God", "Ala", "Brahman", or whatever if you want to, but I don't like to give it a name, because it continues with or without a name we give it. Everything that exists has a direct effect on it's surroundings to one degree or another, so even a dead celestial body (like the moon), though it has no real function as life-supporting system, does have an effect on it's surroundings. For example, it still has a gravitational pull, thus attracting cosmic "trash" from space to it's surface, and even affecting the tides here on earth. I agree with ShutteredEye that the "purpose" of being here (although I know the question is "what is your life's purpose") is to simply BE here, and that's why I include non-living things as well as living things to this. If something wasn't meant to be, it wouldn't. We are living out and experiencing the process of constant physical transformation in linear time from one second to the next. Everything is always changing, and nothing is permanent. Very Buddhist, I know, but that's what makes the most sense to me.
Everything we do has a chain reaction elsewhere in the world, even something as simple as, say, visiting an ATM. Assume you stand at an ATM for a 2 minute transaction. If someone is waiting for you to move on, then the simple fact that you are there using up time has a direct effect on the flow of the person's day. That 2 minutes could just be enough for that person to avoid or to be involved in a car accident later on, for example.