The response takes a long time to generate. The user could just sit there and stare at a blank response, or start reading in realtime as the response is generated.
It doesn't seem to map well tk how I put together a thought either, but admittedly I wouldn't really know how the mechanics of my brain do it, maybe it's not so different just with some auxiliary modules bolted on ha.
tl;dr: It decodes the output one word at a time, but at each step it can focus on any mix of words from the input via the attention mechanism. So the output token n can't depend on future output token n+1 in GPT, but it can attend to any of the input tokens