"High quality" recompresses large files, but gives you unlimited storage space for these recompressed files.
"Original" doesn't recompress or alter the files at all, at the expense of using your Google Drive/Email/Storage space.
I'm not necessarily on board with auto photo backup (I don't completely trust it - some photos should just remain private), but I like that they give you this choice.
What I really (really) hate is that they've moved the delete button. It's no longer a button on the toolbar at the bottom (on the Android app), it's in the menu and labelled "Delete local copy". I take a lot of throwaways while trying to get the right shot - deleting photos is an every-day part of my workflow (I delete more than I keep). This has now gone from one quick tap to three (there's a confirmation dialog when you finally click the delete button). This is a very frustrating change.