One reason could be to keep them silent about certain secrets. For example, if they witnessed something illegal or immoral in a powerful lord's castle, having their tongues cut out ensures they can't spread the word. Another possibility is as a form of extreme punishment for disobedience in a very strict and cruel society within the fantasy world.
Maybe it's a dark magic ritual. In some fantasy novels, dark wizards or witches might use servants with their tongues cut out as part of a spell or sacrifice. The servants' inability to speak could be seen as a way to prevent them from calling for help or cursing the caster. It could also be a symbol of their complete subjugation to the dark magic user.
Chapter 826, "Deadlock", is the clearest early answer because the violence around Yu Li becomes final instead of hypothetical. The chapter turns that into something concrete by moving from The Li Family Master had spoken, so Father Yu quickly followed suit...
Chapter 705, "Is She Her Own Destined Noble Person?!", is the clearest early answer because it turns Chen's situation into a concrete plot event. It opens with He Wen was made to laugh by Bai Lu. and closes on Outside the building, Zhao Lang gazed thoughtfully at the car parked....
Chapter 685, "Defense Value Skyrockets!", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. It opens with The scene went live on Weibo right then. and closes on How could he not be furious?.
Chapter 827, "Mystical Event Investigation Bureau", is the clearest early answer because it turns Qixing Tower's situation into a concrete plot event. The chapter gives readers a real answer by opening on Jiang Li's words indeed make sense.
If you want the clearest chapter reference, go to Chapter 17. Early on, Jiang Li glanced at Jiang Shixu, who was tense and anxious behind her, and...; later, the chapter leans on Okay, let's go eat." After speaking, she strode forward..
Chapter 713, "Uncle Will Owe You for Life", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event. It opens with Jiang Li raised her head and was startled by Zhao Lang's smiling eyes.
Chapter 731, "Ye Chu Died", is the clearest early answer because it turns Jiang Cheng's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Dean, can I really not do it? and follows through with I heard from the dean you haven't been here in a year..
Chapter 790, "Formation Night", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. Early on, In a Daoist temple on the outskirts of the city.. By the end, He is Shen Lan's son after all, and Shen Lan used to be God Jiang's....
Chapter 768, "Sorry Everyone", is the clearest early answer because it turns Uncle Jiang's situation into a concrete plot event. The chapter gives readers a real answer by opening on With a "snap", Jiang Zhao Yuan's purple clay teapot shattered on the...
Chapter 665, "Give You a Good Lesson on Behalf of Your Mother!", is the clearest early answer because it turns Wen Feifei's situation into a concrete plot event.