Alexandre, the "Return as JSON object" relies on you to specify the JSON schema in your prompt. This might be useful for more complex JSON structures that the "Define..." list won't allow you to do. Otherwise, the AI will select the outputs. (Sometimes, this can result in multiple outputs that may not be consistently the same, so be sure to provide a complete prompt when using this option.)
On the other hand, if you have less complicated needs, manually defining the outputs still gives you direct control, you just be clear about the parameters you want to return.
Does that help?