As you can see from this example. Nested contexts can be a useful tool when you want to have a different organizational mechanism for how you structure and separate concerns.
The root concern [when_adding_two_numbers] contains its own establish, because and it blocks.
Each of the nested contexts alters the setup as needed to test a different aspect of the component under a different context. And they also have their own it blocks to make their concern specific observations!!
For information, it is important to note that this is nothing more than a different organizational tool. You can express the exact same tests currently in mspec without needing to use nested contexts.