Image source datawiki.net
Is this to slow down airspeed. I know that the high pressure and low pressure above and below the wing both contribute to downwash, but how does this airfoil form downwash?
Downwash is simply the deflection of air due to airfoil (or wing etc.)
Symmetric airfoils don't produce downwash (or lift) at zero angle of attack.
However, as the angle of attack is increased, the (symmetric) airfoil produces both.
Source: Effect of angle of attack on flow field and body forces by Paul Nathan from youtube.com
Symmetrical airfoils are used in rotary wing applications, vertical tail (and rudder) etc.