I take 2 kinds of photos.
Proper photos which I keep in folders on my Mac. I manage them with an image management app - modify them, add categories, notes, etc, and then synch them to my iPhone and iPad with Finder via USB. 99% of those pics are taken with a proper camera, not a phone. If I do take proper pics with my phone I download them to a folder on mac, import them to the IM app and then delete them from the phone and synch them back to my phone using Finder via USB. I hardly use my phone as a camera. I probably take about 2 pics a month at most with my phone unless I find myself without my camera and something good happens.
Pics I take with my phone are disposable snapshots for future reference. E.g. if I drink a wine I like I'll take a pic of the label so the next time I'm shopping for wine I can see if I can find it. Ditto, say, a nice cheese at a friend's house. If take my lawnmower to bits I might take pics with my phone so that when the spares arrive I can put it back together.
With the previous Photos app these two classes of pics were completely separable on the phone. The proper pics were in the "From My Mac" section, organized by the folders as they appear on the Mac (Decade/Year/Month/Index in my hierarchy). The snaps I took with the iPhone appeared in the "Recents" section of Photos - which used to be called Camera Roll. The 'from my Mac' photos did not appear in the Recents section, even if they were taken recently. So when I'm in the supermarket and I remember the cheese all I did was tap photos, recents, and it would be there - most likely on the first screen with no swiping required because I take hardly any photos with the phone.
Today the nearest equivalent of "Recents" includes every pic I've ever taken. So if I snap the cheese and then go on a holiday and then to my niece's wedding and then go to the supermarket there can be hundreds of pics between the cheese pic and now. Previously it took seconds to find them. Now it takes a lot of swiping and messing about to get past all the holiday and wedding pics I've taken to find the one solitary picture of a cheese label - whether in library or Recent Days - because all the pics in the From My Mac section are lumped together with pics I take with my phone. And that assumes Recent Days works, which it doesn't on my phone.
If you only use your phone and Photos for managing your photography then all of this will either be a mystery or a surprise to you because you'll never have needed or known about this separation between Mac pics and iPhone pics.
My use-case is not unusual; many of the people complaining about the new photos are complaining about the removal of this facet of Photos. Other users who do use their phone as their main camera but don't use Photos on the Mac used to be able to use this separation as a means of identifying which images still needed to be imported for processing before deletion - that's become very difficult for them and if I managed a photo collection that way I'd be tearing my hair out. It's made the photos app a real pain in the neck for us and there's no good reason as far as I can tell.