Metaphysics - Book VII

At the end of book VII in Chapter 17, after he's shown us how all the previous answers of "what is thinghood" were inadequate, I think he concludes that (a?) thinghood is the source of an independent thing that maintains itself.

I still feel kind of empty about this though, so here's my question (unless you'd rather correct what I just wrote):

Philosophically, what is the significance of this, and can we do anything with it, other than not get into the kind of philosophical missteps that the are being shown in the previous chapters of Book VII?

So Aristotle presents this impasse at the beginning of chapter 5: "if one denies that a statement that adds things together is a definition, will there be a definition of anything that is not simple but consists of things linked together?"  I still don't understand why this is an impasse at all, or rather why this should be an issue.  Why would anyone deny that a statement that adds things together is a definition?  It seems like such a person would say that composite things are undefinable, yet such things are those which are most worth defining.  What is the advantage of saying that there can be no definition of things that are not simple but things linked together?