What problem does this address?

Changes made to block themes can be exported from the Site Editor. But the export it is not a complete working theme.
If the export was complete, users could create themes without knowing any code.

What is your proposed solution?

The following is missing for it to be a complete theme:

Short term:

• Add an empty index.php file to the export.. Also see https://core.trac.wordpress.org/ticket/54272
• Add a basic style.css file to the export. Example style.css file header with theme info.

These two additional files are all that is missing to be able to activate the theme.

Long term:

• Export theme.json. Site Editor: Add global styles to export file #36619 Site Editor: Add theme.json to export file #39048
• Suggestion: Replace the style.css file with information from theme.json.
• Naming the theme on export Use more specific naming in edit site export files #38194
• Suggestion: Include license file and readme.