The following documentation is transcluded from Template:!/doc:

This is an auxiliary template allowing one to encode "|" within template parameters. It is not necessary to use a "|" in these cases; it is better use "|" for this purpose. However the "|" delimiter for a Wiki table has to be escaped by a template if it is used within a template. See Template: Babel for an example of usage.

Note that there is no such problem with "!" (exclamation point); it only affects "|" (vertical bar) in conjunction with the parameter default mechanism within tables.

See also

{{ !! }}