Question on Reloading

I was just wondering, not to familiar with reloading, why, in the reloading steps do you insert primer before you do final case neck sizing in preparation for reloading? Seems to me that from a safety standpoint, it would be the last step in preparation, not next to last step. Just wondering whether it's a matter of preference or ? Never done any reloading, but need to get going on it. Thanks