As the Joker himself says, the origin may be invalid.
No, the reason it's a must read is because a) it's well written, b) it was the first serious look at the Joker's psychology, c) it cemented the relationship and dichotomy between Batman and Joker, d) it influenced countless stories that came after, and e)it showed Gordon as awesome. "By the book, you hear? We have to show him! We have to show him that our way works!"