A great many photographers only deliver your photos by way of a download. No disks, no thumb drives.
Anyway, you now have your photo files downloaded to your hard drive. Well, hard drives fail, and computers crash. You don't want to lose your photos to a computer issue. We suggest picking up a few thumb drives, they're not expensive. Place your photo on 2 or 3 thumb drives or an external USB drive. Another suggestion is Amazon Drive, Dropbox, or other online services. In my mind, there is no such thing as too many backups.
I keep multiple backups on and offsite.