The reason that the intonational day of forests is important is to inform people about how important forests are important to humans.
Huge rainforests like the amazon can have huge effects on the weather or make there on microclimate.
The UN have certain activities on the 21 such as tree planting and other communal activities.
On average one American uses 317.515 kilograms of paper each year which means many football of forests need to be cut down to meet this demand.
Source: https://www.google.com.au/search?safe=strict&rlz=1C9BKJA_enAU732AU732&hl=en-GB&biw=1024&bih=653&tbm=isch&sa=1&ei=EADTWNL0Acai8QX475nADA&q=+forest&oq=+forest&gs_l=mobile-gws-img.3..41l2j0i67k1l2j0.42803.45640.0.46728.11.11.0.0.0.0.606.4040.2-2j6j2j1.11.0....0...1c.1.64.mobile-gws-img..8.3.1245...0i8i30k1.aVlUQRyYST0#imgrc=mjeg1p7oBtCj2M: