In English, a lot depends on the writer's intended point of view (POV). You can see this most often with the use of the verbs "go" and "come":
Will you go to the party with me? (POV is here, where we are, traveling there to the party)
Will you come to the party with me? (POV is there at the party, even if we are not actually in that location yet)
In your question the author's POV is at the time of writing. It doesn't matter whether the argument was written in the past, or will be written in the future, the perspective is as if it is happening now.
One of the more famous examples of this is Mark Antony's speech from the play "Julius Caesar", by William Shakespeare:
Friends, Romans, countrymen, lend me your ears;
I come to bury Caesar, not to praise him.
Mark Antony is saying that he is here, now, to say something to his fellow Romans. He hasn't said it yet, but his POV is as of the time he will be saying it.
In the same way, in your quoted text the author is saying, "The argument I present at this moment (is for a particular purpose)". In many cases this use is more common than the future tense, especially if the time frame is not important. For example:
I don't say I dislike him because I disagree with him; I say it because he is a dislikable person.
In this sentence, it doesn't matter when said "I dislike him". The point is that my dislike is kind of timeless and ongoing.