We can already set the width for different kinds of fields. Sometimes it makes sense to make even a single field a "component" because it's used across many content types. This could be an "enumeration" field for example, which allows for editing the list of options in a single place, the component, as opposed to a top-level field on many content-types.
However, enum fields are often short, and it creates a very ugly admin panel appearance when the component it's embedded in is forced to take up the full width of the admin panel anyway. It would look much nicer with a 50% width component next to a 50% media field, which already takes up a lot of vertical space and doesn't need a lot of width, for example.
Seems like an easy feature to add. Fields inside will still just take up relative space: 33%, 50%, 100% of the parent component width.