There are good reasons to backup your Mac and make it the center for Photos. David Murphy makes complicated what should be simple:
It’s incredibly safe to delete photos from your iPhone. If your iPhone’s space is filling up, if you don’t like the composition of a shot, or if you’ve offloaded your photos to another service or storage device, delete away. Your smartphone won’t mind.
But where are the original photos? Mac? Or, iPhone?
If you’ve set up iCloud on your iPhone and you’re automatically uploading photos to Apple’s service… then it’s critical to realize that your “synchronization” isn’t a one-way street. Delete a photo on your iPhone and you’ll remove it from iCloud the next time your device has an internet connection.
I know many iPhone users who don’t know that, so maybe deleting photos isn’t as safe as Apple wants you to believe.