Lawrence - before iOS18 there was a thing in Photos called Recents that I could hit and see only the pictures that I had taken with my iPhone. None of the other pictures, synched from my Mac, could be seen - they were in a different set of folders called "From My Mac". Some users, including me, clearly found this very useful.
This folder called Recents is no longer there. There is a folder called "Recent Days" which has every photo on my phone inclucing those synched with my Mac - which is not useful for me. The Library also includes every picture on my phone. There is now no way to easily isolate the pictures taken on my iPhone from all my other pictures and this is a bit of a pain for me and the way I use my iPhone photos. The camera roll in the Camera app replicates this with the important exception that the thumbnails are only visible as a single row, not a grid, which makes it much less useable for people like me who use iPhone and Photos the way we do.
Apple has removed/changed the Recents functionality and that's why users are complaining. The fact that you don't understand why this is a pain probably means that your use-case for the iPhone and the Photos is different from mine and other users. It doesn't mean that users who are pointing out that a major part of the Photos app functionality has been taken away are wrong and should be scolded for complaining about it.