This makes it easy to group options within a select, as described in the MDN HTML docs: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/optgroup

There are a variety of requests for this in support forums on ACF's site and WP.org, as well as various janky workarounds like this: https://wptips.dev/how-to-create-acf-nested-select-optgroup/

Proper support would be great!