What problem does this address?

Since weights can be a range, it would make sense for any weight within the range of the registered font face to be enqueued.

For example:

wp_register_webfont(
	array(
		'font-family'  => 'Source Serif Pro',
		'font-style'   => 'normal',
		'font-stretch' => 'normal',
		'font-weight'  => '200 900',
		'src'          => get_theme_file_uri( '/assets/fonts/SourceSerif4Variable-Roman.ttf.woff2' ),
	)
);

Font weight range from 200 to 900 gets registered. But right now, if you try passing a font weight within this range, you get an error:

The "Source Serif Pro:400:normal" font face is not registered.

What is your proposed solution?

Passing a font weight within the range (200 to 900) should enqueue that registered face.

In theme.json:

"fontFamilies": [
	{
		"fontFamily": "Source Serif Pro",
		"name": "Source Serif Pro",
		"slug": "source-serif-pro",
		"fontFaces": [
			{
				"fontFamily": "Source Serif Pro",
				"fontWeight": "400",
				"fontStyle": "normal"
			}
		]
	}
]