Programming

It's the sound a Mid-Westerner makes when they're trying to get past you.

But for real, it's because this was super common:

typedef struct {
   // data structure
} SomeDataStruct;

void SomeFunction(SomeDataStruct * data) {
  // lots of functions like this
}

Like many things, once OOP became a thing, people started adding to it. This is why you get a ton of different definitions and such.