Answer:
I never was, am always to be. No one ever saw me, nor ever will. And yet I am the confidence of all, To live and breath on this terrestrial ball.
Used left or right, I get to travel over cobblestone or gravel. Used up, I vie for sweet success, used down, I cause men great duress.
A harvest sown and reaped on the same day in an unplowed field. Which increases without growing, remains whole though it is eaten within and without. Is useless and yet the staple of nations.
You heard me before, yet you hear me again, then I die. Until you call me again.
It cannot be seen, it weighs nothing, but when put into a barrel, it makes it lighter.
What do angels sing in the shower?