Should the internal disk of a Mac be erased? I saw this post which said that the internal disk of a Mac should not be erased, but it doesn't explain why. Does anyone know?
Yes, you can always erase a disk internal or external to set it up clean. The only reason to not erase is if you want to try and recover or back up any data. 

I might try to repair a disk from recovery mode once or twice, then I erase to get a clean install.