A few things, off the top of my head:
1) He remained faithful to dialectical materialism (as opposed to Deleuze, Foucault, Habermas, Analytical Marxists, etc.) and heavily influences 21st century materialist ontology. At the second half of the 20th century it became obvious that classical Marxist materialism contained hasty simplifications. (See Zizek's rants on Lenin's Empiriocriticism – a work loved by Karl Popper.) How communists understand the world directly influence their praxis.
2) He is crucial for understanding the capitalist zeitgeist. Concepts and problems of commodity fetishism, (class) consciousness, representation, surplus value are at work in Marx's oeuvre. Lacan read Hegel and Marx and was in contact with several Marxists of his time and further developed/updated these concepts, or gave them completely new footings. If you want to understand ideology you can't go around him (or Althusser, for that matter). For class struggle a sufficient grasp of ideology is crucial, for class struggle is inevitably a struggle in ideology as well.
3) His theory of the four discourses is crucial in understanding possible social links, and possible transformations thereof. If you want to understand what distinguishes certain human institutions (parties, universities, workplaces), gatherings, or historical periods (Stalinism, late stage capitalism, revolutionary upheavals) you need not go further. He's certainly "useful" (ech) if you want to think about post-rev institutions and want to be prepared for the challenges we will face, being able to keep the subversive edge of the revolutionary institutions while at the same time keeping them efficient and sustainable.
4) Lacan struggled with his own institution as well. After the death of Freud the International Psychoanalytic Institution became a conservative force, betraying the spirit of Freud's work for the letter. A very similar tendency was at work in Marxist parties. With these two we have a common root: a group of people and their relationship to knowledge, training new cadre, keeping the subversive edge, updating the basics without betraying the project itself. His proposed institution of "the pass" is an attempt at solving these difficulties and provides lessons for radical collectives as well.
5) Psychoanalysis can help you, modern psychology is a tool of bourgeois control (biopolitics). No, really.
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1997 → 2007 → 1995:
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Study help:
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Find a Lacanian analyst:
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