Since linking suspects to crimes usually involves establishing why their personal feelings towards someone would be intense enough to lead them to act violently, it's often incredibly difficult to track down serial killers.
Since they don't necessarily have any connection to their victims or any real reason to meet them, investigators over the years have had to be creative in their efforts to catch these mass murderers.
It's for this reason that it's not unusual to see people try to build a psychological profile for a given killer. The idea behind this being that if they can think like the person they're chasing, they're more likely to predict their movements and catch them.
So it's clear why people would want to establish some commonality between serial killers, but reports of one author's findings do a lot to show how this can be less than helpful if we get carried away.