It may not seem logical if you are just looking at the API. However, if you use Podio you'll see that Podio has no concept of a task without a responsible person. You must select a person responsible for each task.
The API could of course throw an error if you don't specify a responsible, but in this case we see it as a convenient shortcut. This way you don't have to look up your own user id to create a task for the currently active user.
Does that make sense?