Suppose that you download (or transfer to Dropbox) all your photos on Google Photos. Now you want to delete them from the Google service. How are you supposed to do this? To delete photos in the web app, you have to literally click on each individual one to select it. There appears to be literally no way to mass select all your photos to remove them from Google Photos. The Google answer to thousands of people apparently asking how to do this is "you can use the storage management tools to remove some types of files". That is the Google wizard which helps you remove large files, blurry photos, etc.