Wello has already a complete API available where all actions are available. But when you want to connect on it, you need to authenticate as a real user of the domain.
In case of ServiceDesk, it's not a user that is connected but a contact. And we want to be sure that security is applied : the contact should only see data related to companies he linked too.
In short, it gives access to all action available via ServiceDesk, without the need to use the interface of ServiceDesk.
If one company want to insert directly Service Request in the system of one customer of Wello, he can do that using the ServiceDesk API instead of using the ServiceDesk website.
But the system is open and available for everything !