lets assume I have used a assets directy from the URL like you said
https://www.xyz.image.png but then the webpage goes down for over a week and the asset will completely dissapear from my project also.
Likewise if its a image link I found from a quick google search, you can never say if it will remain there forever. if the image author removes it then my project will look akward.
and also Thanks to Glitch, I even code from mobile now-a-days,
and I use I-phone, from which it is hard again to download the image and upload it.
These are the basic cases for which I suggested this idea