Stupid question, why do we air beds?

[Easterchuck]

You know in the morning, throw back the duvet, remove pillows etc.
Why?

The sheets feel so much more fresh when you get into bed at night, if you've aired the bed.

The sheets feel so much more fresh when you get into bed at night, if you've aired the bed.

The sheets feel so much more fresh when you get into bed at night, if you've aired the bed.

The sheets feel so much more fresh when you get into bed at night, if you've aired the bed.

The sheets feel so much more fresh when you get into bed at night, if you've aired the bed.

Sorry. I obviously really wanted to emphasise just how much fresher the sheets were.