The everyday use is format. One approved frame becomes a 16:9 header, a 4:5 feed post and a 9:16 vertical without recomposing or reshooting, which is why a crop matrix belongs in the pipeline rather than in a scramble the day before launch.
The limit is that outpainting invents. Extend far enough and you are generating a room the art director never approved, so extensions get reviewed like any other generation, not treated as a resize.
QUESTIONS PEOPLE ASK
How do you get multiple aspect ratios from one AI image?
Outpaint from the approved frame rather than re-generating per format, then check each extension for invented detail before it ships.