2) He tells us important stuff about how humans think and act.
For Conrad, I believe, what goes on inside a person's head is at least as important as how they act in the world. Perhaps more important, because understanding motive is the key. With out understanding motive all action, even terrorist acts, are random. I believe Conrad is correct when he exposes characters as being at the whim of their own emotional needs; he gets deep in the heads of anarchists, spies, policemen and quiet little housewives and shows that they all pretty much are driven by the need to feel secure, or to be protectors, or have their egos stroked. Many characters believe they are acting in selfless support of a cause (be it anarchy or rule of law) but ultimately all are driven by impulses they are probably unaware of. And so, dear reader, are you and me. Conrad was not the first to make this observation but he presents it in such a way as to make it really strike home. I'm sure there is a lot of other important stuff in the book too, but this was just the main one that I
felt.
3) I love the internal stuff - which probably is just a personal thing. I love Chapter Six, in which a Chief Inspector of the police is conversing with his superior. Between each line of dialogue Conrad gives us paragraphs of internal thought - in short he makes a mockery of that "show, don't tell" rule that is supposed to apply to good writing. Conrad tells everything, and it works! This should bore me to death, but it actually stimulates my thinking. Later in the novel (Chapter Eleven), this dragging out of the action by all the interior stuff raises an already tense scene to an almost unbearable level of tension. It is incredibly effective, but I fear too many busy modern readers just don't have the patience for it.
4) It seems very relevant to today. That's one sign of a great book, isn't it? This isn't some dusty classic that explores issues only related to historical events. It speaks to us, now, in a voice that is urgent and vital