

Simply put, because you often want to change the state of something without breaking all the references to it.
Wild off the top of my head example: you’re simulating a football game. Everything is represented by objects which hold references to other objects that are relevant. The ball object is held by player object W, player object X is in collision with and holds a reference to player object Y, player Z is forming a plan to pass to player object X (and that plan object holds a reference to player object X) and so on.
You want to be able to change the state of the ball object (its position say) without creating a new object, because that would invalidate how every other existing object relates to the ball.
Oh it won’t end