From Joomla! Documentation
It contains usage information, categories and other content that is not part of the original template page.
To view the template page itself, see Template:!!.
This is an auxiliary template allowing to encode "
||" within template and parser function parameters. To get both symbols in final output this template is unnecessary, better use "
||" which is rendered the same. Use this template in parameter values of templates and parser functions of which the result is used in table syntax, since this requires the "real" pipe character.
This template cannot be used for putting thes characters as text in a wiki table; in that case, again, "
||" can be used.
Note that the table syntax code "
!" (exclamation mark) can be used directly in parameter values, so for that a template like this is not needed.