Among the many definitions of the word 'purpose' there is one that refers to mere instrumental value.
The purpose of the radiator on a car is to cool the engine. The purpose of the “Cash for clunkers” program is to get gas-guzzling, polluting clunkers off of the road (thus reducing air pollution and gasoline consumption).
This is a definition of 'purpose' that I certainly do not expect us to eliminate. This definition does refer to something real.
Furthermore, this definition of 'purpose' is particularly relevant to those who think that humans were created by a divine entity. In the same way that we create a radiator or a watch to serve a purpose, if God created humans, then humans must have a purpose as well.
We also see this sense of the word 'purpose' when people talk about "being an instrument of the Lord." Such a person will likely speak of the value of "being a part of something bigger, more important, than myself." Such a person views himself as a tool. His self-worth depends on whatever value he might find in being a part of something larger. If he is not a radiator in some divine automobile, then he is worthless lump of matter.
Purposes or ends in this sense certainly do exist. This blog serves a purpose. It is the instrument through which I have decided to discharge a promise I made when I was in high school to leave the world a better place than it would have otherwise been.
As a desire utilitarian, I hold that the only ends that exist for a true statement of purpose to refer to are determined by desires. The "purpose" for all things in the universe is to realize states of affairs in which P is true for those who have a desire that P. This blog serves the purpose of fulfilling (I hope) my desire to leave the world a better place than it would have otherwise been.
Where there is no desire, there is no purpose. Where there is no desire, a purpose-statement is false.
My actions have a purpose – to realize states of affairs that fulfill the most and strongest of my desires.
My desires might even have a purpose. Some of those desires were molded by people who had reason to promote in me those desires that would fulfill other desires. In this sense, some of my desires have a purpose of tending to fulfill other desires. This is why others engineered a society in which I was more likely to adopt a desire for truth, a concern for the well-being of others, and an interest in leaving the world a better place than it would have otherwise been.
My life would have a purpose if it turns out that a God with desires created me. That God would have acted so as to fulfill the most and strongest of his desires, given his beliefs, and would have created me for the purpose of fulfilling the most and strongest of those desires. We would still have to ask whether those desires had any merit. Are they good desires or bad desires? This would reflect on whether my life as a created being served a good purpose or a bad purpose.
Here, again, we enter into the question of whether having a purpose is a good thing. There is no value to be found in the mere fact that life "has a purpose" in this sense. We have to ask the question of whether that purpose is a good purpose. The direct leap many people make from "having a purpose" to "having positive value" is wholly unjustified.
Life is certainly used for the fulfillment of desires – some of which are good desires, and some of which are bad. In a sense, I am using my life as I write this blog posting – something I suspect that I would have difficulty writing if I were not alive. I keep myself alive so that I may continue to realize states of affairs that (I hope) fulfill the most and strongest of my desires. In this sense, we can say that my life has a purpose – in the sense that it has purely instrumental value.
However, there are two points to make with respect to this type of purpose.
The first is that a divine purpose or desire-independent purpose does not exist. Anybody who claims to have “found a purpose for something” who has not found a use other than the fulfillment of desires that exist is mistaken.
The second is that having a purpose is not necessarily a good thing. Auschwitz had a purpose in exactly this same sense. So did the attacks on 9/11. A person who “has a purpose” in this sense of the word cannot immediately rest content with the knowledge that his life has value. He still needs to answer the question as to whether the purpose itself is good or bad. Assuming, of course, that it really does exist.
When a person speaks about his life "having a purpose" in a sense that means "necessarily having positive value," this is not the sense he is talking about. Or, if it is the sense he is talking about, then his reason circuits have shorted out.