The Pre-Socratic philosophers are very important for understanding the origins and foundations of Western thought in general. In their work, it is possible to glimpse the way in which philosophy and science arose out of (i.e., away from or in contradistinction to) mythical thinking through a series of "foundational insights": that "thought and being are the same", that all being is one, that all appearance is constituted out of a unitary, irreducible principle or component, that all is in flux and that no thing repeats itself as such, that every thing is number and has its own number. It's probably worthwhile to get a good explanatory text about them if you're interested. Since their work has persisted only in fragmentary form, the way in which they are interpreted by others has a lot to do with our understanding of them. A grasp of Pre-Socratic thought helps one to understand not only Plato and Aristotle, but also Hegel, Nietzsche, Heidegger, Cassirer, and Gadamer, among others -- all of whom re-inserted ancient Greek thought into modern philosophical discourse in one way or another.

In my opinion, it is relatively easy to differentiate philosophical work that references only contemporary ideas from work that recognizes and pays tribute to the overall historical tradition of philosophy. Many concepts used in contemporary philosophical arguments, that today are naively taken in their concrete meaning as somehow self-evident and justified by science, have their origin in purely ideal postulates that, in their original, Pre-Socratic form, would strike most modern thinkers as bits of quasi-mystical nonsense. However, in many respects, our modern way of thinking is the manifestation of goals set out by the Pre-Socratic thinkers.