Why is there air?

1. If there wasn't air there wouldn't be anyone to ask why there's not.

2. Gas is lighter (less dense) than liquid and solids and as such it
forms the top layer of this ball called earth.  Most planets have some
type of gas/liquid/plasma surrounding them...ours just happens to be
Hydrogen, Oxygen, Nitrogen, Carbon and a few other components that
seem kinda abundant on our lil rock.

-Fractl

The original atmosphere on Earth was made up of hydrogen and helium,
those being the most abundant gases in the cloud of dust and gas that
formed into the planets.  Hydrogen and helium are hard to keep hold of
for a planet as small as Earth, so they were quickly lost into space. 
They were replaced by gases released from the Earth's crust via
volcanism, and water vapour.  Some of this water was already in the
Earth's crust, but more was added by comets colliding with Earth.

So now we have an atmosphere that's mostly carbon dioxide and water
vapour, with a bit of nitrogen. But carbon dioxide dissolves in water
to form carbonic acid, which reacts with some rocks to form
carbonates, and so the ammount of carbon dioxide in the atmosphere is
reduced.

And then life arrives on the scene and everything changes again.
Photosynthesis breaks down water and carbon dioxide and releases
oxygen.  Fractl said (tongue in cheek) "If there wasn't air there
wouldn't be anyone to ask why there's not".  That's exactly wrong -
there is air because we're here asking about it.

Ian G.

Bill Cosby answered this question on his first comedy LP album way back in '64 (?)

I remember because I was nine years old, and I wondered the same thing

He was a Phys Ed major at Temple University, and he said, of course,
it's so you can blow up volleyballs and footballs!

ts

Factl gave a good answer only slightly toungue in cheek. We = humans
modify the atmosphere only slightly and deserve little credit for the
existance of air. iang also gave a good answer except the last
sentence.   Neil

We = Life

I do tongue in cheek too ;-)

Ian G.

Air exsists, because elements exsist.  Even if you only have one kind
of element (like hydrogen) you will always have it in different forms
(like liquid hydrogen).

Air or the Atmosphere exists primarily because certain conditions are
right around certain celestial bodies with certain elements.

It's use of course besides the fact that it contains the element
oxygen (vital to our survival) is also it's pressure, being more
condensed the closer to the celestial body.  In retrospect, it's like
the supposed "aura" of a person (or perhaps the Magnetosphere would
be).