The blank line between the two actually – well, I suppose some clients might act differently than others, but certainly in the Web UI on lemmy and IIRC Reddit – produces a different effect, has a larger horizontal space, and is intended to be a paragraph break rather than just a line break.

foo  
bar

(with two spaces trailing “foo”) Gives:

foo
bar

And

foo

bar

Gives:

foo

bar

It’s not normally a massive difference, but suppose you’re writing poetry, say, you’d probably rather have paragraph breaks between verses and line breaks after each line in a verse:

Stopping by Woods on a Snowy Evening

Whose woods these are I think I know.
His house is in the village though;
He will not see me stopping here
To watch his woods fill up with snow.

My little horse must think it queer
To stop without a farmhouse near
Between the woods and frozen lake
The darkest evening of the year.

He gives his harness bells a shake
To ask if there is some mistake.
The only other sound’s the sweep
Of easy wind and downy flake.

The woods are lovely, dark and deep,
But I have promises to keep,
And miles to go before I sleep,
And miles to go before I sleep.