Answer:
A prologue is used to give readers extra information that advances the plot. It is included in the front matter and for a good reason! Authors use them for various purposes, including:
Giving background information about the story. For example, in a sci-fi book, it may be useful to include a description of the alien world, perhaps in a scene that illustrates its essential characteristics and functioning, so as not to confuse readers by plunging them into a completely foreign world in the first chapter (and having to explain it then or leave them lost, which may lead to disinterest).
Grabbing readers' attention with a scene from the story. The author could pick an exciting scene from the middle of the story to draw readers in and make them want to keep reading.
Describing a scene from the past that is important to the story, such as a fire where the main character's father is killed, which is the motivation for the action in the novel.
Giving information from a different point of view. The story is written in first person, and the prologue is in third person. The prologue focuses on a secret of one of the characters (which the main character would have no way of knowing, and the author would not otherwise be able to tell the reader due to the first person perspective).
Expressing a different point in time. For example, the prologue may be about the main character who is in her eighties and who is remembering her childhood, which is when the story takes place (and which begins in Chapter 1).
