These keyboard shortcuts are also used in emacs; I'm not sure if that's the source of their inclusion in MacOS or if they have a common ancestor between them.

What's the difference between "just after the most recent newline" and the start of the line?

The difference is when the current paragraph of text has line wrapped. Home on Windows goes back to the start of the current line, Ctrl+A on Mac goes back to the start of the current paragraph.