It is there and you can use it as per your needs and in combination with other options, like show title, category title, etc.
For me it adds flexibility. It is good to have it there by default, because there are so many scenarios that something like this might be needed and it adds extra control and options. I remember once I was working on another e-commerce platform and needed something like this for the categories pages. I had to create it. If also Joomla didn't provide this, I am afraid that I would have to add this almost in every website I build.
If and how both of these will print in the html, depends on the template and any possible component overrides.
In many cases (templates), the article title prints within an
<h2>, while the page heading prints within an
You have then the possibility to set the article title to not be displayed and use the page heading from the menu item, to create your desired SEO title.
It is also useful for categories pages, where your categories may have short titles, but you can set a Longer SEO optimized title for the page heading, as well as for other components where you can create a unique page heading.
Usually, it is wise (SEO concerning) to check how a template through its overrides uses and prints them, and adjust your settings / workflow, or modify the overrides to achieve the best possible combination for your needs.
If you don't need it, just don't use it. It is not necessary that everyone will use each joomla feature on every site.
This question is also about titles and headings, and you might find useful the information provided there...How to change the title tag?