One word: lightning. Look at the warranty on your surge protector. It specifically excludes lightning strikes to your local electricity network. Ask me. It happens.
So I got a nice roomy external drive. Then one day my air conditioner shorted out and blew the fuse and everything else, including my hard drive. No sweat. Bought a new hard drive, took my external out of the back cupboard and – good to go.
I don’t know how this works with laptops, but of course there’s rain, spilled coffee 😉 curious kids, thieves and all sorts of other good stuff.