Okay knowing that I dare to say that it is a bit unnecessary.
If it was unnecessary, it would not be there!
Certainly, the cost of the part, the costs to design placement and to mount it to the motherboard, and the extra weight it adds would all be eliminated if it was unnecessary and not included. And reducing costs and weight are always two primary goals of every laptop maker.
Its only purpose is to provide some rigidity to the substrate underneath.
Exactly - and that is an important purpose too. It ensures the motherboard does not flex and twist when gravity and other external forces are exerted on the laptop case.
For a PC motherboard, no big deal - it just sits there for almost its entire life. But laptops are constantly being moved about, turned vertical, then horizonal, left, right, up and down and flat. In some scenarios, they are even abused - that is, they are banged and tossed about or accidently dropped.
This ring ensures the CPU and its cooler do not move, and the cured bond of the TIM (thermal interface material) is not broken - a good thing.
So the ring certainly does mean something - it is just nothing to worry about.