Adding & Removing Elements

Question

Why the stress removing, Adding, and Replacing Elements using Javascript DOM/Events

Answer 1

For example in a game where spaceship shoots bullet to take down aliens.

Answer 2

Bricks and ball game
Shooter game
(Here you see that we have to check for both adding and removing elements. Sometimes we can also add a feature that if the enemy has very low health then another look is revealed that involves replacing)

And many other uses in a website also!
Like in google! There are show less and show more options!