I am starting to learn Ember.js and don't quite get it why an "outlet"

For example, in your example, the list of scientists can be displayed, so why an “outlet” there?

The outlet will render any nested views. I don’t know exactly what example you’re referring to, but if you don’t have any nested routes under the one you’re referring to, you can safely remove the {{outlet}} tag.

ah, the scientist example is this one: Quick Start - Getting Started - Ember Guides

Since the scientist route is nested under the application route, Ember will render its content inside the application route template’s {{outlet}} directive.

(from the linked page)

If you don’t render an outlet in the application route none of the nested routes (meaning none of the other routes in the application) will render. Outlets are important for determining where in a parent route the child routes will render (for example within a wrapper or something)

{{outlet}} means “render my current child route here”.

It’s designed this way because the parent route should be able to cause the child route to render in a particular place, but without letting the parent route influence the state of the child route, since it’s not possible to pass arguments through {{outlet}}. That helps ensure that each route’s state comes directly from each route’s URL parameters, so your app will be a “good web citizen” with URLs that really represent the main application state.