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This is a really good question but also complex. Check out this excerpt that I think handles this question well. I'll source-link it below; you might want to read the whole thing.
Spiegelman's opus succeeds because he emphasizes that the comic is a comic, but, even so, his father, Vladek's story is true. What makes the book so convincing as a Holocaust story is that Spiegelman recognizes the limitations of his art form and chooses a heavily stylistic rendering of his father's story over a blatant attempt at realism that would ultimately cheapen his father's experience. Taking a highly realistic approach would perverse his work, as his choice of medium would seem totally inappropriate if not shockingly irreverent.
In an effort to theatrically "break the forth wall," Spiegelman makes the stylistic choice to portray his personal writing and drawing process for the book. The process is so dramatic that it even comes across as more of a journey than the stereotypical book brainstorming production, making it the perfect skeleton for a comic book because it is action-packed. This story-telling device of incorporating the author within the text reminds the reader that all of Vladek's horrifying adventures really happened.