why does Qt Designer creates extra unnecessary waste code in PyQt5?

I've created a simple form (without any functionality) which includes 2 widgets (a Push button and a LineEdit) first by manual coding then by using Qt Designer. I don't understand why Qt Designer creates so many extra codes that seem to be useless (to me) and makes it so complicated!. This is the code i've written manually:

import sys

from PyQt5.QtWidgets import QMainWindow,QPushButton,QApplication, QLineEdit

class Example(QMainWindow):
        def __init__(self):
            super().__init__()
            self.initUI()
        def initUI(self):

            btn1 = QPushButton('Button1', self)
            btn1.move(30,50)
            btn1.resize(btn1.sizeHint())
            le = QLineEdit(self)
            le.move(100,100)
            self.setGeometry(300,300,290,150)
            self.setWindowTitle('Event Sender')
            self.show()

app = QApplication(sys.argv)
ex = Example()
sys.exit(app.exec_())

And this is the output of Qt Designer UI file (as a python file)

from PyQt5 import QtCore, QtGui, QtWidgets


class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(417, 297)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.pushButton = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton.setGeometry(QtCore.QRect(60, 130, 75, 23))
        self.pushButton.setObjectName("pushButton")
        self.lineEdit = QtWidgets.QLineEdit(self.centralwidget)
        self.lineEdit.setGeometry(QtCore.QRect(210, 180, 113, 20))
        self.lineEdit.setObjectName("lineEdit")
        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(MainWindow)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 417, 20))
        self.menubar.setObjectName("menubar")
        MainWindow.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "Event Sender"))
        self.pushButton.setText(_translate("MainWindow", "Button1"))


if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
    MainWindow = QtWidgets.QMainWindow()
    ui = Ui_MainWindow()
    ui.setupUi(MainWindow)
    MainWindow.show()
    sys.exit(app.exec_())

Here are my Questions:

1. Can some one please explain to me what is going on with the Qt Designer created file? It seems to me that it creates a seperate class just for the UI and then creates an instance of the QMainWindow class and then modifies this instance within the Ui_MainWindow() created instance. But in my code, i directly created a MainWindow class which inherits from QMainWindow and then set the UI up within that class

2. Also what's the point of passing our QMainWindow instance here??: self.centralwidget = QtWidgets.QWidget(MainWindow) Why not self.centralwidget = QtWidgets.QWidget()?

3. What is the point of these many extra code?!