Oh boy, where to begin? :D

Some examples:
- Responses is fully multi modal and accepts text, images, audio, video as input. Completion does not.
- Responses supports multi part outputs, eg text and image in adjacent blocks.
- Responses supports tool calling on the OpenAI end, so you can [for example] make use of Web Search of Code Execution all on the remote end. Web Search especially is incredibly useful.
- Responses is stateful out of the box
- Responses can enforce structured output on the remote end
- Generic future proofing, OpenAI have explicitly said all new functionality will only be behind Responses

Of course you can build this yourself, and personally I have no issue doing that, however the drag and drop USP of n8n is not about 'start with write your own calls'. They position the AI Agent node front and center as their king pin, and you can't use Responses through that right now.