Even when you sync files - you should be able to delete them on the fly. I have a large movie file on my iPad that’s been there for years, which when on the road I often wish I’d remembered to delete when I was at home, cos gee - I could sure use that space right now.
I appreciate the “putting onto the device” aspect of “syncing” - yes that makes it easy to select content to copy over… but then being unable to then remove the files from the i-device other than via iTunes?? To me, it’s not user friendly.
I guess in most regards I’m no better than my 70yo parents with regard to their understanding of iOS. I tend to believe it should “just work” like with a Mac. Instead it tends to be much more the case of - “you need to know how it just works”. Same as when I deleted all my contacts due to choosing the wrong option with the Cloud years ago.
Following above - I connected the phone to iTunes. It was not ticked as having anything sync’d. I clicked to sync, then unticked, which triggered the “sync” button to hilight. So I clicked it… but then my wife had to rush off before I had a chance to see if it remedied the situation. I’m just hoping I haven’t wiped any recent photos…