Many users do not compress or resize their images before uploading them into a post, so source images can often be a lot larger than the settings in /wp-admin/options-media.php.
Many theme and plugin authors do not respect the default settings in /wp-admin/options-media.php and often do not create custom sizes for things like gallery sliders.
The result is huge-ass images on pages and a slower internet.
WordPress provides 3 default image sizes and allows theme authors to create custom sizes as needed.
Does anyone know how to force WordPress, themes and plugins into using defined sizes in /wp-admin/options-media.php and/ or custom sizes created with add_image_size?
I've seen a few posts on here about deleting original source files, but it seems to me that leaving the original images on the server is nice a reference and fallback for re-cutting with Regenerate Thumbnails later if you need to change themes at a later date.