How is an understanding of the robber's MO valuable in investigating a robbery?

"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.