The usual handling of paragraphs in wikitext causes single line breaks to be ignored, and double linebreaks to be interpreted as the end of a paragraph.
This behaviour isn't convenient when dealing with material that incorporates hard linebreaks - for instance, poetry. You can mark a block of content as containing hard line breaks like this:
"""
This is a line
and this is a new line
while this is yet another line
and this is the final one
apart from this one
"""That renders as:
This is a line
and this is a new line
while this is yet another line
and this is the final one
apart from this one
... and the underlying HTML is:
<p>This is a line<br>and this is a new line<br>while this is yet another line<br>and this is the final one<br>apart from this one<br></p>