How do you clone an internal drive to an external one, and why?
Why do I have to clone the internal drive to an external drive and then reboot from the external drive? What does this accomplish? 


Technically, it allows you to re-partition and format your internal drive as APFS because you can't be mounted to the drive/partition you want to modify.  Booting from the external drive allows you to modify the internal drive.

Realistically speaking this is not necessary because...


The install of Mojave/Catalina will auto convert your SSD from JHFS+ to APFS "on the fly" during the install process
You can accomplish the exact same thing by simply booting from Recovery ⌘ CommandR or the USB installer of Mojave/Catalina (I prefer this method).


In terms of backing up your drive, having a clone of your drive isn't a bad thing, but Time Machine will get the job done better.

As a side note... you don't need SuperDuper or CCC (Carbon Copy Cloner). You can use the built in command dd

$ dd if=diskX of=diskY bs=1M


Where diskX = your source (internal) disk and diskY = the target (external) disk. This command will do the exact same thing as those programs.