What is Build Points?

Hi!

I'm +/- new to this card game, but I've look at lot about infos on it! And there's one thing I don't really know what it is: Build Points.

It's number on the upper-right corner of an Attack Card. I found nothing about it in the Online Demo Instruction Booklet, except of it's name (even in the Glossary they don't tell it). Since, in the Demo Decks, the number are from 0 to 3, I guess it has it's importance, but, well, I don't know what it does...

Can someone tell me? Or is it kinda useless?