Em dash is similar to the letter C in English.

Both of them are irreplaceable.

E: There's a joke stating that the letter C can be replaced K and S (but "ch" can't be replaced by "kh" or "sh"). In a similar fashion, em dashes (commonly used to add relevent information like parentheses or to introduce a definition) has some of its usage overlapped with parentheses and colons, but still has some unique usages that can't be replaced.