The br element is used to force a linebreak. Most br elements are likely to be inserted by WYSIWYG editors in CMS systems, as it can be difficult to make them properly use p elements, without producing invalid HTML.
Some WYSIWYG editors also abose the br element to provide WYSIWYG capabilities, but we recommend not using br for such purposes, unless really necessary.
Users generally have a poor understanding of webdesign, allowing them to "hack" a certain design from a WYSIWYG editor is not recommended, as it can be difficult to predict how such pages are rendered in different browsers.
<p>Line 1<br>Line 2</p>