‘Regular’ film is negative film. It is created to have light transmitted through it for printing or scanning. (This is why you can’t ‘read’ your images off of negatives–it’s a ‘negative’ image.) Slide film was created to be projected–that is, to have light transmitted through it so that the image could be displayed directly. So your image in slide film appears on the slide film much the way your eye saw the image when you took the picture.