The Dark Summit is an auxiliary body of the Dark Council, and exists to assist the Dark Council with the administration of the Dark Jedi Brotherhood. Where the Dark Council consists of the commanders of the Dark Brotherhood, the Dark Summit further includes those in auxiliary command positions and advisory positions.
As the Brotherhood has several parts, there must be volunteers in order to work with the council. For example, the Headmaster, being in charge of the Shadow Academy, works with Professors and Docents in order to carry out a more stable institution of learning; even before that, the Headmaster worked with Eclectic Pedagogues. Another example is the Chamber of Justice, while Hands of Justice are picked by the Justicar in order to act as prosecutor and defense attorney to each accused DB member in order to make each case a fair trial. As Consuls are technically part of the Dark Council, it can be safe to say that Proconsuls, as Consuls' second-in-commands, also work as part of the Dark Summit. And finally, even assistants to Dark Council members, known as Praetors, have their own assistants, called Magistrates, to carry out tasks in case the Dark Council member or even the Praetor to that member is unable or unavailable to do the same.
What the Dark Summit reminds each DJB member is that the Brotherhood is a group effort, and as such it must be broken down in parts for leaders to work with. Assistants to each administrator of the Brotherhood are important in order to not only keep the club stable, but also to improve the Brotherhood as a whole with fresh ideas or projects.