Is there even a difference between the two?
My thought is that "purpose of" refers to an intended function of an item. "Purpose for" refers to how an item will be used, regardless of that use's relation to the item's intended function.
Hmm. I'm not describing that well. How about this:
The "purpose of" a shoe is protecting your feet. A possible "purpose for" a shoe is to smash bugs.
So "purpose of" describes a property or capacity of a shoe, where "purpose for" describes what might be done with a shoe.
This is an interesting topic. To add to bikeboy389's response: the purpose of something is the reason it exists. The purpose behind something is the reason in the mind of whoever was responsible for it. My intuition is that a purpose for something is more likely to be a use or reason associated with it after the fact, or subject to debate.
Found via Google:
Providing Students With a Purpose for Reading
i.e., giving students a purpose/reason to read; and
following God's purpose for our lives
i.e., an individual/agent (God) has designated a purpose for something. (The purpose of our lives would frame it as inherent rather than designated, I think. *God's purpose of/behind our lives would be ill-formed, but the purpose of God for our lives is OK in the right context.)
(There are also contexts in which you can use the construction purpose to a thing, e.g. He embraced religion in the hope that it would reveal the purpose to our lives. This feels fixed or archaic compared to the other options.)
You'd say a/the cause of something to refer to its cause. Having (just) cause for an action, though, is a different construction: it means you have sufficient grounds/justification to perform that action, esp. in a legal sense: The judge's flawed instructions are cause for an appeal.
On the other hand, I don't think reason works in the same way: e.g. a/the reason for getting up early and a/the reason to get up early are fine, but *reason of getting up early sounds wrong to me. You could also say the reason for/behind the decision. Words behaving similarly to reason: rationale and motivation.
For me, it feels right to use of when purpose is the subject of a sentence or a clause:
The purpose of a hack saw is to cut through small pieces of metal.
And, also for me, it feels right to use for when purpose is the object of the sentence or clause
This is an odd-looking tool, but I'm sure we can find a purpose for it.
EDIT: On the other hand, it just occurred to me that it would also be fine to say:
This is an odd-looking tool we found at the dig site. I wonder if we'll ever figure out the purpose of it.
When you use "purpose of", the reader is expecting an abstract rationale. When you use "purpose for", the reader is expecting a more concrete example. Compare:
"The purpose of this website is to educate."
The abstract idea is "to educate".
"The purpose for this website is to get people talking about English."
Concrete: "get people talking". Saying "purpose of this website is to get people talking" in the last example wouldn't sound quite right, although it would still be grammatically correct.
It seems this does rely heavily on the user's ear. I don't see any answers that fully resolve the issue.
I'd like to clarify that I have the sense that "purpose for" is favored in academic expression (jargon, virtually) related to education.
I attempted to involve the definition of the prepositions in resolving this, but gave up before I reached any firm conclusion. Here however, are some (limited) definitions from Merriam-Webster online:
of: 2 a —used as a function word to indicate origin or derivation: a man of noble birth; b —used as a function word to indicate the cause, motive, or reason: died of flu; c :by: plays of Shakespeare; d :on the part of: very kind of you; e :occurring in: a fish of the western Atlantic.
for: 1 a —used as a function word to indicate purpose: a grant for studying medicine; b —used as a function word to indicate an intended goal: left for home, acted for the best; c —used as a function word to indicate the object or recipient of a perception, desire, or activity: now for a good rest, run for your life, an eye for a bargain.
Does the function of "for" to "indicate purpose" make it redundant - or illogical - to pair with "purpose"? Does the function of "of" to "indicate the cause, motive, or reason" give it clear precedence? Or does the function of "for" to "indicate an intended goal" provide a valid distinction - and support the "website" examples distinguishing abstract from concrete expectations?
If anyone can sharpen the focus based on this, I'd be very interested in seeing the scholarship or reasoning.
The purpose OF something [describes the author’s or inventor’s reasons why the object was created. A description OF its purpose (reason for being)]. Ex: The purpose of the Constitution is to form a more perfect union.
The purpose for something [describes the user’s reasons for using the thing for its intended or unintended use. A description for its use. (reason for use)] Ex: The purpose for the Constitution is to enable the Courts to keep the federal Government in check.
That is why OF and FOR are interchangeable whenever the description of its Reason for Being and the description for its Reason for use are the same. And explains why an author uses his or her “ear” when, in context, the thing’s reason for being differs from its reason for use.