Sometimes the strongest antiemetics in the world don't work because the body will do what it needs to do whether we like it or not or whether we think it's necessary or not. It doesn't make sense that we would have a bodily function that isn't (however rarely) necessary. That's like saying we have body parts that are not necessary. Sure, we can live without things like our appendix, tonsils, etc, but that doesn't mean they don't serve an important purpose in our bodies.