It's obvious how it helps on the initial fill. With the spout blocked very little liquid gets up in there, so no significant amount of fat.

Now how it helps on subsequent fills is a subject for someone's fluid dynamics paper. The spout slot is never exposed, yet if I forget to put the plug back in I get more oil in the spout than if I remember.

When you put the plug back in, it drives most of the liquid out of the spout back into the bowl, which probably has something to do with it, but I'm not sure what.