homepage – What does choosing a Post Page do?

I have always been confused by WordPress’s homepage setting, even though I have read some SO answers and WordPress front page logic cheatsheet.

First of all I understand the different between “latest post” and “a static page”, so this is not the problem. The problem is with two types of “static page”. It seems to me if you have set a page for “Homepage”, then the setting of “Post page” is of no use and can be ignored. Is this understanding right? Also I check some of my sites and found that when “Homepage” is not selected, no matter what page you set for “Post page” the Front page will remain the same.

So I’m confused here. Why will choosing a “Post page” be useful?