What does choosing a Post Page do?

Why will choosing a “Post page” be useful?

Because if your homepage is a static page, where will the main post archive go? It no longer exists, but you can bring it back by choosing a posts page. You do this by choosing a page that has the URL you want, and that URL wll become the posts page, overriding that page.

Note that when you choose a post page, that feature takes over that page, and it is no longer used as a page. You can’t for example give the posts page a page template, or set its content. The posts page feature overrides that functionality and replaces it for that URL.