If you have a file consisting of some text, the file might or might not end with a \n
If you have a file consisting of some lines of text, the file will end with a \n because a line of text (unlike text generally) is typically defined by matching ^[^\n]*\n
Therefore, `echo` with no arguments returns a line of text which contains nothing.
I think you are right, it is a matter of convenience, to type less, as the story hints, but not only for writing to files, also for displaying messages on the console.
According to the book quoted in the link:
Since a command should by default execute its most commonly used function, ... echo appends the final newline automatically.
It may seem insignificant, but this story highlights, in nerd humour, some of the design philosophy behind Unix.
If you have a file consisting of some lines of text, the file will end with a \n because a line of text (unlike text generally) is typically defined by matching ^[^\n]*\n
Therefore, `echo` with no arguments returns a line of text which contains nothing.