I wanted to look at pictures on the big screen of my desktop Mac. Leaving Google Photos in automatic upload on my phone meant that photos would all get uploaded but the ones that I culled would be deleted from Apple Photos but not Google Photos.
Using Google Photos on my phone would let me delete both copies (Apple and Google) but it was much harder to make out differences. 27" screen vs 6" screen.
I was using Google Photos in the unlimited free quality mode so their copy shouldn't match my original pictures.
So Google Photos ended up as a place where I dumped photos as a backup. I bought more storage on iCloud.