That's sorta the point... the init isn't the branch condition. It gives terser syntax to a common patter where you want to scope something into the body of an if statement.

  {
    std::unique_ptr<T> x = foo();
    if(x->something()) {
      // use x
    }
  }
  // x out of scope, destructed

becomes

  if(auto x = foo(); x->something()) {
    // use x
  }
  // x out of scope, destructed

It's makes it easier to read the lifetime of x (an important thing you have to manually keep an eye on in C++ in many cases), and to determine what is in scope outside of the if statement. It's analogous to for-loops vs. doing loops "manually" with while. You're just used to for-loops by this point (if you don't like for loops then ok :).)

C# is also getting (has got?) this feature.