Years ago, when I started decorating cakes,

the cardboard was mostly covered in fancy foils. These days we tend to cover them mostly with fondant.Even when I cover my boards in fondant,

I use the fancy foil as a base. You can buy them already pre-covered but, if you run out of them, it's very easy to do it yourself.
 

For cakes that are mostly informal, I will use the fancy foil base. But when I want my cakes to look elegant, 
I will cover the base in fondant.