Why is it more advantageous to use a header or a footer to insert page numbers?

Because your pages will be automatically numbered, no matter how many pages you have in the document and what changes you make. There are also extra facilities available, like giving the total of the pages in the document. Again this can be calculated automatically, so you do not have to worry about it or change it as the amount of pages in a document changes. You can also put a standard formatting on them and be sure they are in the same location on each page, or have their positions changing on odd and even numbered pages.