You might have noticed that paragraphs can be written without end tags </p>:
<p>This is a paragraph
<p>This is another paragraph
The example above will work in most browsers, but don't rely on it. Future version of HTML will not allow you to skip ANY end tags.
Closing all HTML elements with an end tag is a future-proof way of writing HTML. It also makes the code easier to understand (read and browse) when you mark both where an element starts and where it ends.