It’s always a good idea to have an off-site backup (e.g. in case of fires, robbery, natural disasters, etc). If you prefer to manage them yourself, an option is to find someone else who also needs an off-site backup and exchange disk space. You do your off-site on their machine, and they do theirs on yours. With external HDDs, you can just have someone else hold on to it for you at a different location. You can come up with fancier schemes to reduce the chances of data loss or to make the process simpler if you care to do so.