When I was beginning to learn Mandarin while studying abroad in Chongqing, I was very frustrated with understanding the southern accent. Instead of pronouncing "zh", "ch", "sh" (retroflex consonants), the locals would pronounce "z", "c", "s" (dental/alveolar sibilant consonants). As you can see, Pinyin makes this pronunciation disparity easier to understand for learners and to anticipate the way that many southerners will (mis)pronounce Mandarin. Learning all of varieties of pronunciation in China is still a daunting task with or without Pinyin, but at least it describes this major one pretty well.