Hello Momo, I see you have already read some key information about federations, so I’ll try not to repeat what you already read.
The way you interpret federations is tightly coupled to your view point, necesity and type of user. For example, you mention an analogy with Digital Ocean, and that is fine if you are looking at it as a Cloud consumer from a Public Cloud provider. If you are an administrator and you are using OpenNebula to deploy your company’s private cloud, this is a really usefull and easy way to mange internals like users and groups.
As another theoretical example, if you think about fog computing, there is no infrastructure able to provide fog as a service yet. Still, OpenNebula federations “COULD” be used (with some extra work) to manage fog nodes across different fog layers.
So, my advise, if you already read all that documentation, deploy a quick test scenario and try it yourself. This might be the best way to solve any doubt you still have.