Sartre and existentialism did get me to have a more leftist understanding of society, and I think he is a great author to get people into leftism more broadly. It introduced me to concepts such as intersubjectivity, which made teenage me realize how we are shaped by external factors and gave me the tools with which I started to analyze the social structures in which we live in. Since then I've definitely changed to a more material analysis of the world, but I think existentialism is a good gateway to get there. I do realize that there are parts of existentialism that are not compatible with Marxism, but Sartre did attempt to put the two together in Critique of Dialectical Reason (there was a great Rev Left episode on that topic).