You press the seams to one side so you will not have gaps for the batting to 'leak' through. The gaps are 'teeney' but remember we used to sew by hand. If you are doing a star block, it will be flatter if all seams are 'to the right' then there will not be a bunch at the center.
It is also much easier to press to one side, you sort of gently pull the fabric in one direction and push the iron in the other and you will have nice neat seams that are completely open.
Keep in mind that you really want to press more than iron, in other words, press the iron down but do not push and pull as you may distort the seam, particularly if youhave something on the bias.
Mary