The reason you do it is to get the alcohol out of the engine. Alcohol holds water and water... causes rust. That's the engine part. For the tank, well, the same thing can apply. Remaining alcohol with a vent to the outside can accumulate water. And, it can also drip out on you wife's new carpet.. if, of course, you are married. If you are not, then well, she won't care, will she.

CGr.