Heidegger’s point, if I can be so bold, in a brief thought:
In a scientific worldview (ours), we as bodied creatures are carbon, oxygen, nitrogen, calcium, etc., various other elemental properties. We are oxygen as much the oxygen we breathe, we eat the carbon that goes into our physical makeup. We are the environment around us.
And yet, we are be-ing, we question the meaning of our existence, and we are each aware that we are in possession of our own existence. It is easy to have meaning and morality when there is God, because that proves that the higher in us is part of a higher order to the universe. However, once God is dead (as pointed out by, not originating from, Nietzsche), how do we retrieve an understanding of us as human with a world made from carbon, oxygen, nitrogen, etc.?
This is Heidegger’s question, why he looks into how we engage with the very question of existence: why us? Why something, and not rather nothing at all?