Althusser's theory made a cultural politics possible because it said: cultural apparatuses are actually extended apparatuses of the state, invested with power relations that reflect relations of class domination in the society. It therefore became as valid to be working in a cultural arena as in a parliamentary or an industrial arena, or wherever. One could say, look, there's the Althusserian argument that explains that the way power works in a developed capitalist society resides in a whole network of cultural institutions that somehow secure social relations of production. That validated one's work. But at the same time, it invalidated it. Because, according to the theory, if cultural institutions were Ideological State Apparatuses, on the one had they were valid areas of intervention; on the other hand, if those interventions didn't go beyond the cultural institutions themselves, ultimately to take on the state, then they were merely reformist. Cultural institutions were seen as extended arms of the complex modern state, so unless challenges went beyond them to engage in the supposed common struggle against the state, wherever it was found, they could be accused of merely collapsing back into reformism.
John Tagg, "Practicing Theories" (an interview with Joanne Lukitsch, 1988), in The Critical Image: Essays on Contemporary Photography, ed. Carol Squires (1990)