'Google Drive sync for your Mac or PC' and 'Auto Backup for Desktop'. Google has two features to automatically backup photos i.e.
If you use 'Google Drive for your Mac/PC', then you should not use Google+ Auto Backup for desktop.
But, if you want to backup photos on your computer, then you can use 'Google Drive for your Mac/PC' by turning it ON. Currently, the Auto backup setting is not available using a computer or mobile web browser.