alt Hacker NewsNot just so that you don't lose it in case of an invasion, but it is proof that you would be willing to help out the host country in the case they were invaded. Also, that you would not invade them.
In the olden days, the local lords would marry their children together, to create familial groups and ensure that for the forseeable future the local areas would not have a conflict. Modern democracies don't have such valuable family members, we use valuable objects instead.
The United States and Canada must have enjoyed that feeling of safety, knowing that the Netherlands would not invade them. And of course, if someone were to invade the United States (or Canada), knowing that the mighty Netherlands would come to their help would be immensely reassuring.