Althusser makes possible for the first time a non-subjective history of ideas. The concept of the problematic enables precise definition and comparison of bodies of thought with extreme rigor.
For example, that Cultural Feminism is the same ideology as the dominant sexist patriarchism, with the value signs reversed.
Or, that nearly all of Psychology post-Freud exists within Freud's problematic of The Unconscious, even those currents which believe themselves to be in rejection of Freud's work itself.
Or, that much of "New Age" spiritualism is Neoplatonic, swimming in the underground stream which derives ultimately from Plotinus.