Racks generally are smaller enclosures than a 20gal. Aquariums like that are also made out of glass and the two hides (or more) are meant to keep the snake from stressing because it feels too exposed. Even if you have covered 3 sides of a glass tank, you should still keep 2 hides because of how the space is set up in the tank.
Racks are generally dark in the back so the snake thinks its in a hide already, but keepers give the snake the option of actually utilizing a hide and from waht i have seen there are a lot of snakes that will take the hide.