Babel在将ES6类转换为ES5时使用Reflect.construct的原因是为了处理类的继承。在ES5中没有类的概念，而是使用构造函数和原型链来实现继承。当Babel将ES6类转换为ES5时，它将使用构造函数方式来实现类的继承并且重写类的构造函数，此时就需要使用Reflect.construct来调用父类的构造函数。

下面是一个具体的示例：

ES6代码：

class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }
}

class Student extends Person {
  constructor(name, age, grade) {
    super(name, age);
    this.grade = grade;
  }
}

转换后的ES5代码：

function _inherits(subClass, superClass) {
  if (typeof superClass !== "function" && superClass !== null) {
    throw new TypeError("Super expression must either be null or a function");
  }
  subClass.prototype = Object.create(superClass && superClass.prototype, {
    constructor: {
      value: subClass,
      writable: true,
      configurable: true
    }
  });
  if (superClass) Reflect.setPrototypeOf ? Reflect.setPrototypeOf(subClass, superClass) : subClass.__proto__ = superClass;
}

function _construct(Parent, args, Class) {
  if (Class === void 0) { Class = Parent; }
  var child = Object.create(Parent.prototype);
  Reflect.apply(Parent, child, args);
  return child;
}

var Person = function Person(name, age) {
  _classCallCheck(this, Person);
  this.name = name;
  this.age = age;
};

var Student = /*#__PURE__*/function (_Person) {
  _inherits(Student, _Person);

  function Student(name, age, grade) {
    _classCallCheck(this, Student);

    var _this = _possibleConstructorReturn(this, _getPrototypeOf(Student).call(this, name, age));

    _this.grade = grade;
    return _this;
  }

  return Student;
}(Person);