What problem does this address?

The use case for this feature would be a theme that wants to apply a duotone filter based on the colors of the theme. In this example we can think about the background/text color combo such as here:

It would be great if we could define the preset filter in this way in theme.json:

"duotone": [
        {
            "colors": [ "var(--wp--custom--color--primary)", "var(--wp--custom--color--background)" ],
            "slug": "my-filter",
            "name": "My filter"
        }
    ],

So that when the user changes the background color of their site, the already existing images will mage the new background color too.