The root of the word "Qliphoth" (קלפ) means "husk" or "shell", and the original idea behind them is very much one of obfuscation. The Sephiroth of the Tree of Life are the path we must ascend to reach the divine; the Qliphoth are the obstacles to our understanding of those Sephiroth. Yes, the Abyss must be breached to reach the supernal triad, and the Veil must be pulled back to find Tiphereth, but what if we don't even get that far? What if we're unable to properly understand the energy of Hod? Or even of Malkuth? This certainly can happen (if only on a temporary scale), because the concerns of the material world make it more difficult for an individual to step outside of him-or-herself and access the divine.
I have a mental image which I found helpful when I started looking at the Qliphoth, although for other people it may be useless. I sometimes think of each Sephirah as a lightbulb covered in a thick layer of mud. The mud is Qliphotic energy, which is the product of mundane life and which prevents us from seeing the Sephirah for what it is. Instead of a shining light, we see it as another dirty part of the world, and we attribute to it all of the mundane characteristics of the other things in our lives. In short, we think we understand it for what it is, but in reality we do not.
So for example, we have Gamaliel (which, according to sources that aren't necessarily trustworthy but that all seem to agree with each other, translates into English as "the obscene ones"), which is the Qliphah corresponding with Yesod. And where Yesod was about fertiliy, sexuality (ish), and man's foundation in the material world, Gamaliel is a twisted understanding of those principles. It's a misrepresentation of the energy of Yesod--Gamaliel is all about extreme promiscuity, self-indulgence, materialism, etc. And the idea here is that people will approach Yesod, see casually what it's supposed to be about, and think that they understand it. But in reality, they haven't accessed the truth of it; they only understand the shadow, the husk of what Yesod is meant to be, and what they think of as Yesod is actually the murky, misdirected, even chaotic energy of Yesod.
I hope that makes sense.
There are other ways to approach the Qliphoth, but in terms of direct work with the Tree of Life, this one seems the most practical to me. Each Qliphah is the challenge, the potential misunderstanding, misdirection, and unbalanced manifestation of a Sephirah's energy if practitioners approach it wrongly.
Another approach is a more theological one, and has to do with the enormous (and, as has already been pointed out, highly speculative) ramble I went on in the
thread that Snaut linked earlier. The Qliphoth can be used as a theological tool to explore the problem of evil.
Yes, the Tree of Life has Geburah, which manifests some of what we might call "evil" in the world (or perhaps "suffering" would be a better word). But at the same time, the Tree of Life is an expression of the emanations of a fundamentally omnibenevolent godhead, and therefore it could be viewed as inconsistent (depending on your philosophical viewpoint) to have the Tree of Life be the source of evil in the world.*
And so if you do see this as an inconsistency, the Qliphoth put things back into balance. The Qliphoth are perversions of the energy of each Sephirah--not creations in their own right, but twisted versions of the godhead's original work. And this is consistent with, for example, the Catholic myth of Satan (although it doesn't fit nearly as much into a Jewish theological framework) as the source of evil in the world. In this case, studying the Qliphoth might not help you understand the Tree of Life or enrich your Qabalistic work, but would be a more personal exploration to try to understand and quantify the nature of evil and suffering in the world.
*At the same time, the Tree of Life is also an expression of a fundamentally omnipotent godhead, so there are those who would argue that evil cannot have come from anywhere else, because the Tree represents the energy of the unmoved mover. There are also those who would question the description of God as omnibenevolent, and who would describe the divine energies of the Tree of Life more as a Jungian Abraxas. Like I said. It depends on your philosophical viewpoint.
Anyways. These are just some thoughts on the matter. God knows I'm not an expert, so everything I say here is really just a matter of speculation and conclusions that I've drawn based on my own reading. But maybe I can add a different perspective into the mix.
