Vomiting up ingested harmful substances is an adaptive mechanism that prevents them from passing through the system.

“From a purely adaptive standpoint, it's probably better to vomit up things that aren't good for you rather than to have them pass through your system. Especially if those things are contained in lipids. For instance if you ingest something that's in liquid form because cells, literally every cell in your body is surrounded by a little thin layer of fatty tissue, we call the bilayer membrane, it's a little membrane. Fat can move through fat very easily. And so any bad stuff you ingest can get stuck in your system.”