Of course, in order for a suspect to have a modus operandi (MO), they have to have committed a number of kidnappings in the past. This is not a terribly common thing, so police will not often be able to use a suspect’s MO to capture them.
If the suspect in the kidnapping actually has committed enough kidnappings to have an MO, it can be very valuable in investigating the crime. It may, for example, help you to identify the suspect. If a kidnapping is done in a particular way that is part of a given kidnapper’s MO, it can lead investigators to focus on that specific kidnapper. The other benefit of understanding a suspect’s MO is that it can allow investigators to know what the kidnapper might have done with the victim and what he or she will do next. If investigators know what kind of places a particular suspect likes to use to keep his or her kidnap victims, they can concentrate on searching those types of areas to try to find the victim. If the suspect has a particular method of picking up ransom money and evading the police while doing so, the police can be ready to counteract that method.
In all of these ways, understanding of a suspect’s MO can be very useful in a kidnapping investigation if such an MO actually exists.