Why do they put pine trees on top of newly constructed buildings?

Question

Everytime I see a completed building go up, there is always a pine tree right up on the top floor. Just wanted to know what the reasoning behind that was. Cheers x

Answer 1

to replace the ones they cut down to build another building that we dont need

Answer 2

It is part of a "topping off" ceremony to celebrate the top floor being completed.