To my mind, what's interesting about Marx in these early works is not only how he frames and develops his argument, culminating with Capital, but also how he separates his views through criticism of other thinkers and schools of thought, such as Feuerbach and the Young Hegelians. For example, in the preface to The German Ideology, he outlines his goal to bring the Young Hegelian theories -- which Marx says are tied to God and the "ideal man" -- closer to reality by joining them with the "wretched and real" conditions of the German working classes. Further, Marx claims that Feuerbach is stuck in the realm of pure theory, that he does not see the connection between history and materialism, and therefore never arrives at the true existence of man (46).
In this manner, Marx is asserting the value of his "study" to his peers, although his argument has not yet crystallized. In the German Ideology, he says that to answer the most important questions -- e.g. to what extent to historical conditions affect production, and what part does it play in the course of history? -- we must first study the production process, the engine of capitalism. He begins his analysis by stressing the importance and interconnectedness of each mode of the process -- production, distribution, consumption, exchange -- but claims that production, as the starting point, is the predominant and driving force in the process.
In other words, Marx believes his study of political economy is valuable because, unlike other thinkers, he is attempting to marry theory with the reality of current working conditions: i.e, his analysis of the production process, and how it is affected by changing social and historical conditions, will reveal the strengths and flaws of capitalism more accurately than "pure theory." In Grundisse, reinforces these concepts and ideas: for example, how man cannot be productive in isolation, and how there can be no production without the right conditions, such as labor and capital. Historically, he discusses how "conquering" nations like Britain used colonies like Ireland, India, and America to boost commerce and manufacturing, even using the phrase "for pillage to be possible, there must be something to be pillaged." He argues that in nations like the U.S., labor has become a de-individualized process, a form of creating wealth, rather than (as in Russia, perhaps) labor being an expression of skill and individuality.
Finally, Marx states that capitalism has succeed by forming classes and divisions: Capital, wage labor, and property. Town and country. Production and distribution, and exchange between them. All this has created a social hierarchy: the working class, the middle class, and those who own the property and means of production. In my view, these early writings are ambitious and thought-provoking, and when I read Capital I will be eager to see how much he "proves" or follows through on these ideas.