It is to interject the chained value at the right position in the function.

They write that elixir has a slightly fancier version, it is likely around this, they mean (where elixir has first class support for arity > 1 functions)

But the example suggests that it can't interject the chained value at the right position; if that was the case, the example would've been written as `|> array_column('tags', ...)`.