The necessity of a backup shouldn’t be underestimated. We’ve all been there, that time when you lose photos from a water damaged phone or files from a hardware crash on your PC. Thankfully those days are well behind us through automation mainly. Having Photos automatically backed up to Google or files to an external drive give you some level of protection.
However, backing up to external drives only is not recommended as a long term solution. Combining this with cloud backup gives you peace of mind knowing whatever happens there will be a copy somewhere.