Understanding a robber's MO is important to investigating a robbery because it helps the investigator looks for signs and patterns that are unique to the robbery. One should look at many different aspects of a robbery to determine if there is any pattern if there is a string of robberies. For example, is there a pattern to the area(s) that the robberies occur? Is there a pattern to the type of items that are stolen? Is there a specific time of day or night that the robberies occur? Is there a general route into a building or house that the robber normally takes? Once these patterns are identified, the investigator may be able to begin narrowing down a list of suspects who fit that MO.

"MO" is short for "modus operandi."  It refers to the way that a person habitually does things.  In the investigation of crimes, knowledge of criminals' MOs can help to solve crimes by matching a person to a crime.

If a certain robber's MO is known, then other crimes with the same MO can potentially be linked to that person.  This is not sufficient to get a conviction, but it can serve to point the investigators towards the right person.  Instead of having no suspects, the police can use the MO to establish a number of suspects to investigate.  They can, for example, investigate all robbers who have been known to enter buildings through unlocked second story windows or who commit them by having an accomplice fake having an accident while the robber takes money from the cash register in a store.

By keeping track of the MOs of known criminals, the police can have a way to know who to investigate when a crime is committed using a particular MO.