One point not often mentioned here ...
If you keep all your sensitive personal data inside the iMac and you have a malfunction which requires a trip to a service shop to repair, all your data is available to the technicians while it is there (assuming the malfunction prevents you from wiping the internal storage prior to sending it in).
If you simply have the bootable OS and apps internal (preferably on a SSD), you can remove the external drive (SSD, hard disk, RAID, etc.) and keep your personal data safely at home while they work on your iMac. You can also attach this to another Mac in the meantime to continue working.
Of course, you still need a backup. 😎