The following documentation is transcluded from Template:Curly left/doc:

This is an auxiliary template allowing one to encode "{{" within a template or page that is expected to be SUBST'd.

This, along with {{curly right}} allows for a template to be created and shown on a page as an actual template rather than it also being SUBST'd into the page, which could potentially change the behavior on the template being inserted.

Its primary use is on the Wikia-bot welcome message.