They reassure us. They tell us that indeed the puzzle can be carefully constructed and put back together, that order can always be restored, that chaos does not win the day. This act of reassurance cannot take place without a noble man or woman at the center of the story. A person unafraid to wade into that which we don't want to know about and find the solution that will vanquish evil and restore order. The mystery has evolved in recent decades to be as much an investigation of the investigator as an inquiry of the crime at hand.
Investigators now look inward for the solutions and means of restoring order.
In the content of their own character they find the clues. I think this only bodes well for the mystery novel. It is what keeps me interested in writing them. When I sit down and stare at the blank screen, I contemplate character. I think about what I want to do with my investigator this time.