Why break in a switch statement is important?

break.js

const o = {
  i: 0,
  get x() {
    return this.i++;
  }
};
switch (o.x) {
  case 0:
    console.log("a");
  case 1:
    console.log("b");
  case 2:
    console.log("c");
  case 3:
  case 4:
    console.log("d");
  case 5:
    console.log("e");
    break;
  case 6:
    console.log("You're not gonna see this.");
}