Yep, indeed I struggled with that early in the development of Napkin. Why not just pop over to BBEdit and drop some next text in?
So I tried to design a reason, so to speak.
That's where the slug and time stamp come in -- as well as the standardized formatting (which can get sloppy when you drag-and-drop or type or dictate or mix them all together).
It turns out the fomatting is a big comfort to me. I don't worry about whether or not I have the right number of returns at the end of a note I'm appending.
The slug is helpful and more efficient that constantly citing sources and the time stamp is just gravy.
But clearly those aren't compelling arguments for using Napkin instead of an open document in your favorite text editor.