My completely subjective guess is that it might be to avoid ambiguity around reserved keywords. For example, the `void` keyword [0]:

  void function () { return 42; }(); // returns undefined

It's not something you would usually use in your code, but many Javascript minimizers make heavy use of it, and Typescript must know how to handle any valid Javascript without ambiguity.

[0] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...