There are plenty of novels, particularly older ones, where you might see a prologue before the main story actually begins.
Prologues are a way for writers to hint at the main story in some way because sometimes they feel they need to convey even more information than they can comfortably slot into their main story, especially if the writer wanted to explain certain things that would prove otherwise difficult to do in the main story without making the story overly long, seemingly jarring or by burdening the narrative with info dumps.
Sometimes a writer just can’t construct a chapter around certain information or facts pertinent to the story arc.
Prologues can act to highlight something in the past that now has a bearing on the story in the present, or it may have unknown characters that have influenced the main character somehow, but are not actually present in the main story. This is a way writers can provide the reader with clues about a character’s motivations, by providing clues in the prologue that will be apparent to the reader later in the novel.
The use of a prologue can also be effective for setting up different points of view. For instance, the prologue can be told in a different character viewpoint to the rest of the story, or it might be the prologue is told in first person, and the rest of the story follows in third person.