This is an auxiliary template allowing to encode "
||" within template parameters. It is unnecessary to get a "
||" in these cases, better use "
||" for this purpose. However the "
||" delimiter for a wiki table has to be escaped by a template if it's used within a template, see also w:Help:Conditional tables.
Note that there's no such problem with "
!!" (exclamation mark), it only affects "
||" (vertical bar) in conjunction with the m:Help:Parameter default mechanism within tables.