|Impermanence is symbolized by the sand mandala.|
The theme so far seems to be that nothing in the universe, without exception, has a fixed "essence" or self. All entities are conditioned, constantly in flux, arising one instant, and fading away in the next. And while you might agree that this realization is profound (or you might not), you might also be asking what, if any, are the practical implications of it.
As it happens, there is a salient and immediate consequence that follows directly from the "theorems" proved in Zen Preliminaries I, and Zen Preliminaries II. The corollary states that suffering arises directly as a result of craving and attachment, because ultimately these behaviors are irrational and will always lead to an undesirable state of being.
The truth of the claim can be derived from observing that pleasure is not immune to impermanence. Pleasurable sensory experience is just another kind of entity within the universe, and just like everything else, it is empty and it arises dependently on a web of interconnected conditions. Thus clinging to sensory experience, or experience of any sort for that matter, is tantamount to clinging to something that is perpetually in motion: It ensures that our future selves experience dissatisfaction by building our happiness on a foundation of quicksand. Put another way, attempting to hold on to sensory experience is similar to trying to hold on to the flow of a river; no matter how intense our effort, the water will always slip through our hands unobstructed in much the same way that experiences slip through our minds. We destine ourselves for failure and disappointment by fixating on a task that is impossible and Sisyphean.
It's worthwhile to note that the "self" discussed in Zen Preliminaries I is just like the river of experience mentioned here (some including myself would argue that they are identical). Just like the river, the self is constantly in flux, never the same at any two distinct points in time, and its path is entirely determined by the landscape through which it runs. Most importantly, much like the river, the self will also slip right through our hands if we attempt to hold on to it. Consequently, attempting to cling to the self will result in the same kind of suffering as attempting to cling to other types of empty phenomenon like sensory experience.
Clinging to the self also gives rise to the fallacy of "mine" and "yours". This divides resources and compassion unnecessarily, and creates potential for future suffering. Moreover, this fallacy also generates the belief that there is an "I" to experience various kinds of material pleasures. This obfuscates the truth that these experiences simply arise within and as part of the conscious stream, and not as separate entities to be experienced by some observer "behind" the stream. This latter fallacy is perhaps the progeny of all lower forms of desire, as it justifies the craving of pleasant experiences and material comfort.
In traditional Buddhism, this type of craving is believed to be the primary impetus for the cycle of birth, death, and rebirth. Because the false ego craves existence--the argument goes--it literally cannot let go of the world, and thus forces itself back into reality after death. Of course, we need not believe in anything supernatural to see the benefits in letting go of our cravings for experience. We need only perform a cursory glance inwards, and see that all instances of suffering arose either from being separated from object(s) of desire, or being in proximity with object(s) of anathema. In either case, the most immediate agent of suffering was craving; whether it was craving for a more pleasurable experience, or craving to avoid a less desirable one.
While Buddhism may posit unwarranted assumptions about the nature of reality (such as reincarnation), at least it also provides us with rich imagery to visualize some of its more nuanced concepts. Buddha's well known Fire Sermon analogized attachment to fuel and clinging to fire. By letting go of our attachment, the Buddha argued, we can extinguish the fires that torment us throughout existence. Note that the term "nirvana" literally means to "blow out" or "extinguish", a subtle but revealing point to keep in mind when studying Buddhism.
As an aside, what, in my opinion, is beautiful of and unique to Buddhism, is that it applies this idea of deep non-attachment even to itself. In Buddhism, we are meant to view all beliefs and practices (collectively called the Dharma) as a raft whose only purpose is to see us safely across a turbulent sea to a tranquil shore, and as something that should be let go off once the destination is reached. Put more bluntly, Buddhism is to be viewed as a tool to achieve peace and happiness, and not something to be consumed by, as this would be counterproductive to its goal. Buddhism is a self-constraining religion by its very nature: the only way to be extreme is to not be extreme at all.
Pragmatism aside, I hope that I've shown enough cause for any reasonable person to see craving and attachment as irrational and futile and as things that should be eliminated, at least on a purely intellectual level. Realizing the true implications of the empty nature of experience and clinging on an emotional level takes much more time and commitment, and requires practice to fully cement.