What if the worst happens and your account gets hacked and you can no longer access Google Photos? Hopefully, you can regain access to your account, change your password and set up two-factor authentication but you might still have lost some photos or videos in one way or another. If you've freed up storage on your phone, for example by deleting your photos locally, then the only copy will be with Google Photos. It may also be sensible to download your Google Photos library for other reasons as well. It might be a cost-cutting measure to save on the Google One fees or perhaps you're planning on moving to Amazon Photos or Apple iCloud and want to back up your images there. It could be that you have a few different reasons to download your photos from Google Photos.