It’s mostly a caching thing, otherwise you’d have to re-render the images at smaller sizes on the fly which would be extremely computationally intensive.
You’re right that backups can probably skip this part though @stephen and I know @sam has mentioned this in the past? The only downside is restoring a backup becomes very CPU intensive as the backup must then re-render all the resolutions, but that’s not a serious downside in my book – unless you’re in a hurry, or have a CPU underpowered server perhaps?