I explained this issue in detail here: Understanding uploading images via URL / CSV
In short: images added to collection lists (via CSV) can only be used inside collection lists. If I want to use the image somewhere else (outside of an collection list) I have to upload the same image again.
It would be nice (and data saving) to automatically add images that are uploaded to collection lists in folders in the asset panel.
I built a site-builder back in the day and what we did was for any images collection to be displayed as both a grid (for re-arranging) and a list (for titling and alt-texting). We used to kick out unique internal links to each image that could be embedded anywhere else in the site without uploading it again.