A store with 800 products has thousands of images. Alt text written one at a time never finishes,
Alt text is a scale problem — most catalogues have several times more images than products, so a per-image workflow never completes.
Good alt text describes what the image shows, which is also what accessibility requires and what machines can use.
Rules-based generation works for the long tail, but the products that matter commercially deserve hand-written descriptions.
Decorative images should have empty alt attributes, not descriptions, and this distinction is routinely got wrong.
Describe what the image shows, specifically. "Navy wool overcoat photographed on a model, showing the length at mid-calf" is useful.
The full guide has the formats, the deadlines and the worked examples.