when your board slides on the snow, the friction it creates produces a thin film of water between it and the snow.
it is this that allows you to slide along... the water basically lubricates the base to allow you to slide.
wax is 'hydrophobic' which is to say that it not only slides on water, but it actually 'pushes it away'. thus increasing your slide.
putting on wax but not scrapping it means that your board is covered in tiny bumps; which can trap molecules of water and snow and make it more difficult for them to slide off; basically your board has a greater area = more friction = more sticky = less slidey.
the wax enters the holes and pores of your base, to reduce this area and to push water away. any extra needs removing; hence scraping
it does nothing to protect the base, but works only to move water away from it.
(as far as i know
)