Probably echoing what others have said, but here's an article with a salient section:
With all these disadvantages, and hardly any advantages to speak of, you might be wondering if hiccups serve a purpose at all. Well, some scientists have argued in the affirmative.
They point to the fact that even human fetuses hiccup, long before they’re born. In fact, the diaphragmatic spasms are more common in infants than in adults. It’s possible that this reflex helps prevent fetuses from breathing in amniotic fluid while still in the womb; likewise, it could prevent newborns from choking on milk while breastfeeding.
And still others have proposed that hiccuping in the womb trains a fetus’ respiratory muscles for all the breathing they will have to do after birth.
But humans aren’t the only animals that hiccup; pretty much any species that breathes exclusively air — including all mammals — can suffer the same fate. (Birds and reptiles, on the other hand, get a free pass.)
In fact, that’s the reasoning behind another theory, which posits that hiccups are merely an evolutionary “leftover” in mammals, dating all the way back to our fishy ancestors. When these species transitioned from gill-based breathing in the water to lung-based breathing on land, while still possessing both organs, a breathing system that allowed them to quickly close the glottis and direct water only to the gills was beneficial.
We see a similar process play out on a smaller scale when tadpoles grow up and transition into frog-hood. And that may not be a coincidence; believe it or not, the neural patterning that generates a hiccup in humans is almost identical to the neural patterning involved in respiration in amphibians.