All you see is a small star - when you mouse over it you see the text with no explanation. At least for single monitor systems this is confusing. I am using it in a VM and even in dual monitor it seems to do nothing, but might be a VM issue? Expectation: better explanation... and grayed out for single monitor systems. Reproducible: Always
There can be no primary screen, so technically the star is correctly enabled even on single-screen setup. However I agree that from user perspective this is confusing and basically wrong. Anyway, this is fixed in Plasma 5, where we reverted to widget-based controls and primary screen is configured through a combobox which, even on single-monitor setup, will contain "No primary screen" item, which makes it clear.