We should before distinguish about what is the purpose of the Republic.
Without this missing point we cannot pinpoint the real context. Some sources I cannot recall at the moment( sorry, maybe neoplatonic) say Plato kept correcting the Republic books all his life. In any case is such a long book where it is a pertinent assumption Plato did not leave to case the initial sentences.
Several reasons for that:
1) Plato dialogues present meanings not always evident initially,
nothing esoteric, just a fact, knew people that spent 20 years only on
the Repubblic for instance.
2) The book is too long to not take
attention to the first phrase, even because in the 10th book also
there is a noticeable end.
3) The semantic analysis of all his
dialogues show that the way they begin is almost always referring to
some particular context.
Said that, the purpose of the book is political, metaphysical, on
Dike, the justice? How Plato considers when he writes his book, the
environment of that period?
With all these premises we can also think that he is referring to going down to the Ades. And also that can still remain valid other meanings, as the ones quoted so far. The Piraeus is an haven, a place where there are people usually different by philosophers. As it is a fiction, why that procession? It was quite popular this Thracian deity for sure at the time of Plato, but could we think that there was some reasons behind? Bendis as Artemis is linked to Cybele, and here should be seen the link with the last book of the republic where there is the myth of Er. Andin this sense would be interesting to see how the similarities with the concept of a cathabasis in Ades become much more familiar.