Hundreds of Ambassadors applying to be your brand ambassadors can be a little daunting, and going through each profile one by one can take a long time. Unless your brand is looking for a very specific set of ambassadors, it's better to have as many as possible and bulk approve new applications.
This way you can promote brand awareness and engagement internationally, and with anyone who wants to work with you.
However, you do not have to share your missions with absolutely all of your Ambassadors. Missions can be restricted to particular filters or segments which complement the mission's task, and so rewards won't be going out to the wrong people.