An example would be something where you have a metafield defined for a shop and want to render the corresponding HTML and liquid code in the metafield on a template page:
<div>
{{ shop.metafields.app_page.content.value }}
</div>
The value for shop.metafields.app_page.content.value would contain HTML and liquid such as this:
<p>
View product details for: {{ product.title }}
</p>
If product.title in this example is set to: Product title, the resulting HTML should be rendered on the page:
<div>
<p>
View product details for: Product title
</p>
</div>
The use case for this, is to provide users the ability to dynamically add content at their own discretion. This can somewhat be done using theme app extensions and app blocks, but due to major limitations on the size of folder contents within the theme app extension framework this only works for limited use cases and lacks ideal flexibility.
A metafield definition would not fit this use case as this data would be defined once and not per product or per resource.