As mentioned in Send your first fleet sending fleets require 2 transactions. One will make your fleet leaves its origin planet. And the second need to be performed when the fleet reaches its destination.
While you can definitely come back to the game to perform the 2nd transaction, it is possible that the time is not suitable for you. We provide thus a service that will resolve the 2nd transaction for you.
In pas version, you had to go through a registration and top-up process, this is not required anymore. You pay for the service when you send your fleet. THis make a much better user experience.
It is also possible to submit fleets to the agent-service after they have been sent. But this need to be done before the fleet reaches destination.