1

Let me rephrase the question:

What are some benefits of having a single actionable event on screen (e.g. single child folder, single button occupying the whole screen estate, etc.), immediately followed by the user-induced activity (e.g. double clicking on a folder on Finder to see its contents, clicking on an actionable button in an application, etc.)?

To illustrate the question, I've taken an example from the file structure every OSX application seems to have:

Screenshot showing the option "Show Package Contents"

Step #1: We're going to click on "Show Package Contents" to see the contents of "App Store.app".

Screenshot showing only content, "Contents" folder.

Step #2: After the click, we realize there's only one folder, "Contents". This seems redundant, because the users most likely had decided to see the contents of "App Store.app" in Step #1 already.

Screenshot showing .app's contents of "Contents" folder.

Step #3: Here are the real "Contents" of the application.

2

It makes no sense in theory, but can be highly justified in practice.

The Apple bundle case

One example of practical justification is provided by Apple:

While the Contents directory might seem superfluous, it identifies the bundle as a modern-style bundle and separates it from document and legacy bundle types found in earlier versions of Mac OS.

Now let's delve into more details.

The theory

You have raised an extremely important point when it comes to structuring, where this rule is next to unchallenged:

If a parent has only got one child, either replace the parent with the child, or remove the child.

Examples include:

  • Don't have a menu item that only has one sub-menu item.
  • Don't have a chapter in a book that only has one sub-heading.

Reason being, that a parent (or container) is created as a grouping entity for other elements - but a group must have more than one member, or it cannot be called a group.

To illustrate, this menu hierarchy:

  • Products
    • Cars
    • Motorcycles
    • Bikes
  • Services
    • Repairs
    • Trading
    • Sales
  • Get in touch
    • Contact us

Should really be:

  • Products
    • Cars
    • Motorcycles
    • Bikes
  • Services
    • Repairs
    • Trading
    • Sales
  • Contact us

The practice

Classification consistency

It is often important (IA comes to mind here) that all items on a particular level has the same classification.

Consider the following:

  • Cars
    • Fiat
    • Skoda
    • Ford
  • Bikes
    • Peugeot

The first alternative is confusing:

  • Cars
    • Fiat
    • Skoda
    • Ford
  • Peugeot

the second is little ideal:

  • Cars
    • Fiat
    • Skoda
    • Ford
  • Peugeot bikes

Future usage

Whether a shopping list, or a folder hierarchy, it is well possible that initially a container will only have one item. Like in this case:

  • Development
    • Desktop
    • Mobile
    • Web
      • My first site

The Web folder may have one sub-folder, but it's all logical and some folders may be introduced in the future.

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