They need to ensure that any combination of characters will produce some result.

Quite useful in situations such as:

> (str.match(regexWithGroup) || [, null])[1]

I.e. if the regex matches, then give me the first group (1st index) otherwise give me null.

Because you can create that array anyway, like so:

  var arr = [];
  arr[0] = 1;
  arr[1] = 2;
  arr[3] = 3;

so there's not really much downside to also allowing a literal syntax for the same thing.