Diphenhydramine also acts as an anticholinergic which slows the gut motility and contributes to antiemesis and drowsiness.

Drowsiness, as HowlerKarma, is also a symptom of severe allergy, anaphylaxis, and masking that is very bad. In mild nausea being able to sleep is nice. The drowsiness component is the reason diphenhydramine was used as a sedative adjunct in some Anaesthesia but it has hangover and other side effects that newer agents don't, so I have never seen that done.