I missed another reason for a bootable backup: to have a platform that can run macOS installers on (I've had little luck with the USB flash drive installers), and to have a platform for pushing a clone of data back to the other machine.
I do keep all three types of backups, plus more. For my main machine, I have:
- Time Machine
- Secondary Time Machine, connected once a month
- SuperDuper! bootable clone, updated weekly
- Second SuperDuper! bootable clone, updated monthly
- Backup to cloud via Arq, holding data back for 5 years
- 2 SuperDuper! data-only copies, backed up monthly (alternately), with one stored offsite. The backup date is 2 weeks later than the other monthly backups.
Apparently I have a FOLD (Fear of Losing Data).
I think it dates back to when I had a great scheme for using Retrospect to backup my Mac to DDS tape. Everything was backed up weekly, rotating between 3 tapes, where once a month one of them was sent offsite and the 13th oldest tape added back in. So I was covered, completely.
And then my PowerMac couldn't start up anymore. So I bought a new iMac.
That's when I discovered I couldn't connect my SCSI DDS tape drive to the new iMac, which had no SCSI ports!
(I did mange to get the PowerMac to boot
one last time and grab all the data off it. But I swore NEVER AGAIN will I be without a means to recover my data.)