The point of theoretical practice

So here's one question that lingered for me out of our in-person conversation about Althusser yesterday: what, for him, is the point of theoretical practice? He clearly doesn't assent to the Weberian notion that the production of knowledge through the disciplined application of a value-commitment to some empirical field is, in some sense, an end in itself. Indeed, since he's so rigorous about keeping theoretical practice separate from empirics, it can't even be the case that the elaboration of conceptual objects is useful in illuminating some particular aspects of the world; I think that would contaminate the conceptual purity of his "science" too much, and render the line between science and ideology (in his terms, not necessarily in mine) ultimately meaningless. Plus, he's no pragmatist, and "a theory is useful to the extent that it illuminates things" sounds way too much like Dewey or Rorty for Althusser to readily assent to.

So what's the point of the exercise? Marx sits in the British Museum, elaborates a conceptual object called "capitalism," shows how this conceptual object depends on the notion of surplus-value…and the point is what, exactly?

How is this not just theology? (Not that there's anything wrong with that -- from my perspective, anyway. Althusser, I suppose, would think differently, given "opiate of the masses" and all of that.)

Is this just a more subtle form of the Enlightenment Project, with the Truth now imported into a non-empirical realm of Pure Types and Concepts?