- Who is specifying the need for this image size?
The person who wrote the code adding it
- What is this image size going to be used for?
Core doesn't know that, and it's not such a simple question to answer, but the person who added the image size knows
- When will this image size be used?
The developer might have a use case idea, but nobody can be certain, afterall I might add an image size for a particular use, only to find my users really like the way it looks right aligned in a post
- Why will this image size be useful?