We talk about that on the episode, that in some way the Orbs have the essence or spirit of the Prophets and that is what makes them "work"
Finally had chance to listen to this one. It is fascinating, isn't it, to ponder all these things that almost by definition have no answer.
One aspect of the Orbs that we see in the books is what happens when all the Orbs get together. As Unity
shows us, it turns out that when all nine Orbs are opened together all at the same time, they can effectively open a new entrance to the wormhole, or at least to the Prophets' continuum that lies within it.
That's something that obviously couldn't have happened on the show since we didn't get all the Orbs returned on the show. But it turns out to be vital to the resolution of "season 8"'s plot, as it returns Sisko from the Celestial Temple and swaps him for the parasite matriarch, thus saving Bajor (again).
And follow the thread backwards - it was only possible because of all the things that had happened previously in "season 8", thus leading us to questions about whether the whole thing was predetermined to save Bajor, or a handy set of coincidences, or it just looks like predestination because the Prophets can see everything that happens... all sorts of conundrums that are raised by the whole concept of prophecies.
- All 9 Orbs are there to stop the parasites and bring Sisko back from the wormhole
- Because Yevir had gone to Cardassia to find the Orbs and bring them back
- Because Yevir had visited Kasidy and been inspired by the jevonite figurine
- Because Yevir was in confusion over the Attainder he inflicted on Kira
- Because Kira had released the Ohalu book that was found at B'Hala
- And Kasidy only had the figurine because Jake had been working at B'Hala
- Because Sisko had gone to the wormhole
So, Sisko was only saved from the wormhole... because he'd gone to the wormhole.
*mind = blown*
(Of course, I realise all of this is Lit-DS9 rather than TV-DS9, but it is on the subject of the Orbs themselves, and these days the Lit and the TV tend to blend together in my mind anyway.)