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Used to include a pipe character as part of a template argument. Before this was added as a magic word, many wikis implemented this by creating Template:! with | as the content.
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.
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.