Why, why, why
I think most of us are under the impression that the purpose of the plot, in a crime/mystery story is to uncover whodunnit, and of course as a reader I want to know who the guilty party is. But that's not enough. I mean, I don't want to be in the drawing room with all the other players only to be told it was the ladies maid and nothing more. I want to know why.
In fact the why of behaviour is something that generally interests me, it's fascinating to see how past experiences and emotions play into that.
I want to know what was motivating the culprit, even if it's as something as straightforward as revenge, I want to read about how the incident made them feel, the years spent searching for the person responsible, how they inserted themselves in the inner circle, waiting for the exact moment to strike.
But it's not just that person I'm interested in, I like it when there is misdirection, witnesses that tell half-truths, not driven by the desire to protect a murderer, more to protect themselves and whatever secrets they're trying to keep under wraps.
I've read books where the victims all seem random, unlinked in anyway, but there's still a reason they were chosen, or their location was chosen. Perhaps the antagonist of the story gets fired and then starts killing people on their way to work because they're angry that these people have jobs and they don't. Perhaps they kill their boss first, who knows.
The point isn't to make me the reader feel empathy with the culprit, although that can happen; in Murder on the Orient Express (spoiler alert) Hercule Poirot does feel empathy for all the people who kill Rachett, it is because of this, and because Rachett, himself is far worse a criminal, that the Belgian detective allows a theory of an unknown assassin. He understands the pain inflicted on them by Rachett and even though he is such a proponent of the truth, he accepts that in these circumstances it's not the most important thing.
So, why does it all matter - because I want to make sense of it all, that's why.