Currently, typo3 core only allow to set width of images without any layout relationship.
This ends up into providing images far larger than client would have needed (think about a picture in a 6 column layout rendered at 1200 / 600 / 300 px while only making 100px). Not only this consume bandwidth, but also energy to downsize images in the browsers.
Providing some care, it's possible to pre-compute real image size in nearly every layout for every breakpoint. Adding full support for correct img srcset and picture tag, and data-attributes lazyload solution, and css background.
The cons :
-Precomputing 5 to 10 images (breakpoints +1) * 2 if retina can be time consuming on first load of the page.
-Configuration to take account of grid system, breakpoints, and content frames.
-Adapting templates to make system aware of layout.
-Not artistic direction friendly (but there is currently no one in typo3)
The pros :
-Less time to load
-Automagic once configuration done, but with standards grids setups it should be ok right out of the box.
Why not ?
Who wants it ?