Large language models are good at writing, so this pattern leverages this core ability to assist in authoring a quality prompt.
While many user types could benefit from having a quickly typed thought refined by AI this pattern leans heavily towards benefiting users that aren't confident with their prompt writing.
Having this inline, instant assistance can even boost some users confidence to start writing in the first place.
The pre set assistance provided will depend on your particular context. Consider what types of writing enhancements would help your users most. Not only for their prompt writing but also to enhance the quality of their generated results. Perhaps they actually need their prompt to be longer with more detailed.