Sartre builds up a big, abstract, speculative system, apparently as a framework for his belief in human freedom, choice, and responsibility. What does this construction accomplish that simple assertions wouldn't of our freedom, our not being determined, our defining ourself via our yet-to-be-accomplished projects, our responsibility rooted in our unavoidable need to make choices? Perhaps both emphasis (you'll be less likely to forget you are free), elaboration (you'll learn more what being free as well as trying not to be implies), and examples (you'll learn more of the ways in which people try to avoid the weight of their freedom).
Even if the experts tell you they have you all figured out, you'll have decide whether to buy that or not. Even if you want to be all figured out and delivered from uncertainty, they (and you) may be wrong. If Sartre only argued for our individual freedoms, he wouldn't be so important. It is in his exploration of the ways in which we cringe from our freedom, of our "bad faith", that he connects and makes what seems a speculative, abstract system instead a powerful emotional truth.
If all this philosophy has captured you, Satre's novels and plays are no less powerful in presenting his themes: the novel "Nausea", the 3-volume "The Roads to Freedom", the play "No Exit", and more. Or if "Being and Nothingness" seems a bit much, try "Existential Psychoanalysis" which consists of two more grounded excerpts from "Being and Nothingness".