If your world matrix is an identity matrix (which will be the case if you are keeping your object unrotated, unscaled, and untranslated/unmoved), the inverse of that will still be an identity matrix, and the transpose of that will also still be an identity matrix.

So taking out that line shouldn't make a change, in the specific case of having your object essentially unmoved from the start point. Once you move your object, the world matrix won't be an identity any more, and neither will the world-inverse-transpose matrix, so things will be different in that case. That's the reason we need this particular line of code.

Honestly, though, that's a great question. Thanks for asking it!