There will most likely come a time when one or both parents are interested in taking their children away on an international vacation. International destinations are looked at a little differently than a destination within the United States. The reason for that is that some countries are not a part of the Hague Convention.
The Hague Convention prevents children from being wrongfully removed from the United States. So, if a parent is looking to take their children out of the country, they will most likely need the written consent of the other parent and the country must be a part of the Hague Convention. The reason for this consent is not to make it difficult for one parent to travel internationally, the key purpose is to protect the children involved.