This is one of the most common artwork types that I encounter, apart from posters.
Sometimes it is the only artwork available for an event, or has unique artwork or information that isn't contained on the posters.
This is mainly for social media digital banners, a horizontal format image to go at the top of a page. However, I think it would be fine for it to cover 'real' banners as well (e.g. painted or printed banners).

Examples:
https://beta.musicbrainz.org/event/7f4d569e-7c51-4434-b46d-40ed8796eeda/event-art
https://beta.musicbrainz.org/event/6aaf3692-924b-45c0-8ae9-416f74d0f0cc/event-art

Because the 'horizontal' format is very different to most other images that will be added, it would be a useful marker.
For instance, in future we may also want to consider whether we display these in the sidebar by default (maybe it's better than nothing, but the horizontal format doesn't display well).