KH2 beginning is a bit of a (distorted) explaination of what happened in the past two games. However, only if you actually played the last two games would you really understand it. It also explains a bit....
KH1 beginning was a premonition, yes, but I thought it was also introducing Sora. Sora's a loyal person, who's being controlled by forces.... beyond his control. He tries to help Riku (the blue-haired dude) from the tidal wave and stuff, but is swept away and has no control over what happens next.
Its like a beginning theme to an anime or cartoon, it doesn't have to make sense, just look cool XD