1) Anyone else reading your code can get a quick plain English overview of what is happening without the need to study the code.
2) It helps you to document what you have done and by doing this you might notice a mistake you have in the logic of your code through explaining it in plain English
As you can see I have added a list of explanations as to what the code is doing in the screenshot below. The comments are the lines in green:
Remember to put the comments in the appropriate place, don't just list them all at the top of the class. Put comments about a click event in the click event and so on.