When I was a cop, which mostly pre-dated the computer technology we take for granted today, a major investigation might include the creation of a link chart on a bulletin board, showing people, locations, cars, etc, some connected by lengths of string or yarn. Really creative people would use different colours of yarn to show different types of links. I saw announcements from organisations that taught week-long workshops in the creation and maintenance of these charts.
One of the problems in creating one of these charts was that they took up a lot of room. Our detective bureau of maybe 25 officers had a single large bulletin board where such a chart could be posted, and there were often several major cases (plus numerous minor or routine cases) working at one time. The chart had to be covered most of the time, as a suspect or a friend of a suspect might see it and get a leg up on the investigation. All told, they weren't very practical.
The big displays you see on TV programs like Law and Order: SVU reinforce the myth that detectives work on a single case at a time. In shows like these, usually the entire squad is devoted to investigating a single incident or group of incidents. The wall display is a device to move the story along and reinforce the relationship of the players. You're reminded of who the suspects and victims are, what sort of things they did, etc. I quietly marvel at how the detectives have managed to obtain great, well-exposed, high-resolution photos of all the players. Photos like that might be possible to obtain, given enough time and effort, but many of them would be from driver's license files and photocopies of old mug shots, and would vary a lot in quality.
Cops do still use link charts, but they're more helpful in complex, longitudinal investigations where there might be hundreds of people, places, phone numbers, IP addresses, etc. involved. There is now software for managing this kind of information. While you might find one of the simpler board displays in a detective office, it's far more likely this sort of thing will be kept on a computer.
Tim Dees, retired police officer
It's one way of keeping people and evidence (or multiple cases) linked. Some of our detectives operate better with visual connections available. Others work better with lists and spreadsheets. It all just depends on how the detective processes the information best.
Christopher Hawk, 20-year police patrol veteran