Many tags that you see as a begin and end pair can also be given as the singular with the slash at the end. For example, I tried all of these and none gave an error with EpubCheck.
Code:
<p/>
<em/>
<b/>
<span/>
<body/>
As for why, I agree with Tex2002ans, you could use them with a class and a css combinator. (I'd be interested to see useful examples with the em and b singular tags because I can't think of any at the moment.)
Not related to this, but speaking of hr tags and combinators, in my stylesheet.css I have
Code:
hr + p {
text-indent: 0;
}
So that paragraphs after an hr aren't indented. All of my p tags are naked/classless.