What I'd like to do here is to explain, to atheists and theists alike, that the "purpose" of an object has nothing to do with the object itself: the purpose of an object is simply what conscious beings intend for the object. I say this because many people, religious or not, speak about an object's purpose as if there is an objective fact of the matter.
As an example of atheists misusing the word "purpose", I've often heard atheists say that, realistically the purpose of life is to reproduce. However, this is not the purpose of life; this is not what life is "for". This is simply what most life automatically does. I will unpack this distinction below in my "light box analogy" and explain why this is an incorrect use of the word "purpose".
My first piece of evidence for "purpose" being a state of mind, rather than a quality of an object, is Webster.
This definition, and the synonyms listed, indicate that the purpose of an object is simply what is intended or desired for that object (intended or desired by a conscious being), and has nothing to do with the object itself.
To illustrate this further, and to explain why the "purpose" of life is not only to reproduce, as I've heard many atheists argue, I will talk about my hypothetical "light box". Imagine that I am building a "light box", and I tell you that its purpose is to light up. However, as I haven't finished it yet, it doesn't actually light up, not yet, anyway. Furthermore, I also tell you that this box was originally manufactured as a paper weight, and I am now turning it into a "light box". Still, I maintain that its purpose is to light up, and I think we could all agree that its purpose is indeed to light up, even if it doesn't currently light up, and even if it used to do something different. This demonstrates that an object's purpose has nothing to do with what it currently does, nor with what it did in the past: it's all down to what I intend for it.
In this same way, life (as in, every living thing) doesn't have a purpose, unless you, the reader, intend to use every last living thing on this planet for something. Your life, however, can have a purpose: it's whatever your life goals are: it's whatever you (Bill, Jill, Kelly) are "for". You, as a conscious being, can give yourself a purpose.
Another illustration that may help is what I call the "doorstop" thought experiment. Imagine I open my front door and enter my house, and as the door begins to swing shut behind me, the wind blows a rock underneath the door, jamming it open. At this moment, is the purpose of the rock to hold the door open? No, that's just what it happens to be doing. However, if I come back to the door and say, "Oh! Perfect! I needed something to hold the door open so I can bring a table inside," now the rock has a purpose: now the rock is for holding the door open. Notice that while the rock suddenly has a purpose, nothing about the rock itself has changed. The reason it has a purpose (to hold open the door) is because I, a conscious being, gave it one: I now have an intention for it. Thus, purpose is simply the product of a mind (in this case, my mind). This also means that there is no such thing as "objective purpose" by definition: purpose is necessarily subjective because it requires a mind (a subject) to generate it.
The point of this blog post has been made before, but I still think it deserves more publicity in atheist circles, as I still hear the word "purpose" being misused. This point has most notably been made by Richard Dawkins in the context of evolution and the apparent purpose of evolved features of plants and animals.
Dawkins discussing the question of "purpose"