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.
Process termination can happen relatively silently on mobile environments, and apps are expected to save and restore state much more than desktop apps do. That's part of the paradigm that these OSes promote that users shouldn't need to understand or manage whether a process is running or not, especially on the relatively-memory-constrained hardware on which they run on. I'm not entirely sure that I agree with that as a goal, but I have seen people using Windows or MacOS who do not understand the concept of a process running, and never close anything, have a ton of programs open, with their machine slowly grinding along. If you figure that part of the idea behind mobile OSes was to be more usable to people who had trouble with personal computer OSes, it kinda makes some sense. In any event, it's the OS rather than your web browser that made that design call.
In Firefox Fennec, you can close all tabs by tapping the button by the URL bar that brings up the list of tabs and then tapping the three dot menu in that, and choosing "Close all tabs".