how is pressing F9 a form of difficulty?
why do people like design like this? Does it make them feel smart?
It makes the world feel more alive and believable. It provides tension, because you never know what might be around a corner. It provides you with optional "hard difficulty" fight, if you decide to face that foe despite being underleveled, and immense satisfaction when you're clever, and find a way to beat him. It also provies satisfaction when you don't take the extra challenge, but instead leave him alone for now, but then come back couple levels later to wipe the floor with him.
What other design do you prefer? Placing only level-appropriate goons on your path, to be rightclicked to death and make you feel better about your skills? Level scaling maybe?