Question

why does energy need to leave Earth?

Answer 1

The energy is still there in the form of gravitational potential. Think of leaving the earth as a process similar to riding a bicycle up a hill. (When you ride a bike up a hill, you're moving against Earth's gravitational field thereby gaining potential energy, just like what happens to the rock when it moves up away from the surface of the earth.) If the rock is eventually so far away that the effect of Earth's gravity is negligible, this is a similar situation to you reaching a flat spot on the top of the hill that you just rode your bicycle up. Now you can just sit there on your bike for as long as you like, but if you turn around and approach the crest of the hill, you will start rolling back down again, with your potential energy that you gained while climbing the hill now being converted to kinetic energy as you coast back down. This is the same as what would happen to the rock if someone nudged it ever so slightly back toward the earth (it would fall back down).
Escape velocity is needed when we leave earth

Answer 2

We know energy can never be created and nor be destroyed. hence we have constant amount of energy that flows in the universe.

When outside energy gets inside the earth, it needs to get out of the earth too. because it may create an imbalance of energy in the universe.