Why does Art Spiegelman include his much older comic, “Prisoner on the Hell Planet”?

I think Spiegelman wanted to give the reader context, especially about his mother. Prisoner on the Hell Planet (1972), is a short comic strip written by Art Spiegelman covering the events of his mother's suicide. The work is illustrated in a German Expressionist manner, juxtaposed as an insert in his larger work Maus, Vol. I, which uses animals as its main form of characters.