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The difference is that group membership is transitive. The children processes of your children (or of any other process added to the group) do belong to it by default. Knowing that all those processes will behave correctly is substantially more difficult than doing direct manipulation of a list of known processes.

Of course, if the group hierarchy is standardized, processes known to behave badly can be moved out of the group, but it would mean modification of the process, thus (at least partially) negating the benefits of being able to do it without changing code.