We are digital librarians. Among us are represented the various reasons to keep data -- legal requirements, competitive requirements, uncertainty of permanence of cloud services, distaste for transmitting your data externally (e.g. government or corporate espionage), cultural and familial archivists, internet collapse preppers, and people who do it themselves so they're sure it's done right. Everyone has their reasons for curating the data they have decided to keep (either forever or For A Damn Long Time (tm) ). Along the way we have sought out like-minded individuals to exchange strategies, war stories, and cautionary tales of failures.
An internal drive only protects you against drive failure. If the device is physically/electrically damaged or lost, so is your backup.
An external drive can be stored separately, and retrieved using a separate device if necessary.
An unpowered drive will also last longer usually.
I do both.
I have two SSD in both my PC and my laptop. And two DAS connected to the PC and shared over wifi. One of the DAS hold my large media and backups. The other DAS is mostly used for backups.
Every time I boot my PC, or laptop, a new rsync snapshot of the full /home on the primary SSD is automatically created on the secondary SSD.
I also manually create rsync snapshots of folders on the PC/laptop primary on the primary DAS and also of folders on the primary DAS to the secondary DAS. By manually I mean automatically, but only after I trigger it by double click on a script on the desktop.
Use internal drives with a dock and store cold. Saves the hassle of dealing with a bunch of cases and power adapters.
For fully on backup all the time use a versioned backup like time machine or a cloud service.