The subjunctive mood is used to express desires, doubts, the unknown, the abstract, and emotions, the opposite of the indicative mood which is used to express actions, events, and states that are believed to be true and concrete. (http://www.spanishdict.com/topics/show/75)
I love the subjunctive mood (in all languages). Rather than being a state of "unreality" (as some 'experts' suggest), it is properly known as irrealis in linguistics. It is the basis of the language of the great classics.
In the introduction to Common Sense, Thomas Paine begins:
Perhaps ['were it so', so to speak] the sentiments contained in the following pages are not yet sufficiently fashionable to procure them general favour;... [emphasis and note mine]
This is an example of the subjunctive introducing (or, more precisely, framing) something important that follows by posing a 'possibility' adverb, perhaps. -In the case of Paine's introduction:
...a long habit of not thinking a thing wrong gives it a superficial appearance of being right, and raises at first a formidable outcry in defense of custom. But the tumult soon subsides. Time makes more converts than reason. (ibid)
The subjunctive mood is a proven rhetorical device because, basically, the Paine pattern above is general rather than particular to this historical document. It may, for instance, serve as a conclusion:
Your Honors, I derive much consolation from the fact that my colleague, Mr. Baldwin, here, has argued the case in so able and so complete a manner as to leave me scarcely anything to say. However, why are we here? How is it that a simple, plain property issue should now find itself so enobled as to be argued before the Supreme Court of the United States of America? I mean, do we fear the lower courts, which found for us easily, somehow missed the truth? Is that it? Or is it, rather, our great and consuming fear of civil war that has allowed us to heap symbolism upon a simple case that never asked for it and now would have us disregard truth, even as it stands before us, tall and proud as a mountain? The truth, in truth, has been driven from this case like a slave, flogged from court to court, wretched and destitute. And not by any great legal acumen on the part of the opposition, I might add, but through the long, powerful arm of the Executive Office. Yea, this is no mere property case, gentlemen*. I put it to you thus: This is the most important case ever to come before this court. Because what it, in fact, concerns is the very nature of man…(http://www.monologuedb.com/dramatic-male-monologues/amistad-president-john-quincy-adams/)
-ie, 'were it so' this case wouldn't be before the esteemed audience
The subjunctive mood (in English, at least) is subtle and is handled with subtlety in the most effective oration and argumentation. It is not overtly marked (as it is in other languages) but entwines itself into the syntax and grammar of the language, wispy like smoke.