Antimatter navigation

Hello there,

i am just try to figure out how, to build a navigation. There is one thing i dont understand. In the navigation.html.twig of the antimatter theme there is a loop through the page collection like

{% for p in page.children.visible %}

And after that, there is a second loop using the site object. Like this one:

{% for mitem in site.menu %}

But why is this extra loop needed?

Thanks is advance.

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