I think opening "prologue" scenes like this are mostly about priming the audience's expectations. Movies do this too; the story of The Matrix begins with Neo working as a corporate drone, but the opening scene is an action sequence with Trinity getting chased by a police as a promise of things to come. There's value in letting the audience know what kind of story they're in for, especially if you're planning on taking awhile to get there.
Notably, these kinds of scenes aren't there to sell the audience a false bill of goods. Quite the opposite, in fact: the first few chapters of a story aren't always representative of the whole, hence the addition of a prologue to give the audience a sense of what's to come in later chapters.
As an example of what can happen when an author doesn't do this, Dan Wells' I Am Not A Serial Killer is about a teenager who works in a mortuary and investigates a series of brutal deaths around his town, and discovers that the killer is actually a millennia-old monster. The supernatural reveal doesn't happen until halfway through the book, and if you go on Amazon and sort by 1-star reviews, you'll find a lot of them come from people who thought they were reading a crime story and then got upset when they discovered they were reading a paranormal YA novel. When it came time to publish Wells' next book, his publisher told him, "This time, we're putting the monster on the cover of the book."
Notably, these kinds of scenes aren't there to sell the audience a false bill of goods. Quite the opposite, in fact: the first few chapters of a story aren't always representative of the whole, hence the addition of a prologue to give the audience a sense of what's to come in later chapters.
As an example of what can happen when an author doesn't do this, Dan Wells' I Am Not A Serial Killer is about a teenager who works in a mortuary and investigates a series of brutal deaths around his town, and discovers that the killer is actually a millennia-old monster. The supernatural reveal doesn't happen until halfway through the book, and if you go on Amazon and sort by 1-star reviews, you'll find a lot of them come from people who thought they were reading a crime story and then got upset when they discovered they were reading a paranormal YA novel. When it came time to publish Wells' next book, his publisher told him, "This time, we're putting the monster on the cover of the book."