It doesn't affect any functionality or aspect of the output, except it will also break the line in the HTML source. So when you open up your HTML source in notepad, you'll see lines and not just everything sticked after each other on one line.
Makes it easier to wade through HTML code, too, if you're debugging...
It's a good habit to use 'em, but you don't have to (at least not when you use [br] tags).
