Field mappings are rules that specify how the connector transforms data from one system to the other.
Let's say you have company-wide mapping rules. You can create a field mapping based on those rules and templatize it. All field mappings linked to the template will change whenever you update the template. You may also have special requirements for a particular field mapping that's an exception -- you want it to follow the rules of the template except for a couple of specific fields. In this case, you can create the field mapping based on the template and then customize it for those two fields.