In computing we have a CPU, whose job it is to perform generic instructions for general-purpose computing. However, for some performance-intensive tasks, such as graphics processing, there can be performance gains and optimisations realised from designing a processing unit that is specialised to a given task. Although all metaphors are imperfect, this can be usefully applied to the role and function of a second brain system.
Our human brains have many capabilities and responsibilities, including sustaining life. The role of the second brain is to augment our overall system (consciousness?) by providing a special-purpose capability - that of being an Integrated Thinking Environment. It is a location in which the user can partake in the Curation of ideas broadens horizons and helps draw patterns. This is also why the digital garden and concept of An evergreen note is a unit of knowledge must be considered - the integrated environment is composed by the formation of evergreen notes, their ongoing refinement and through connectivity to other evergreen notes.
This isn't to liken the system exactly to the human brain, it is more that A second brain is closer to a GPU than a CPU, in terms of its purpose - generality vs specificity - serving a bespoke role.
A second brain is closer to a GPU than a CPU in terms of its function.