Omnibusses are mostly a marketting move.
They make sense in a lot of cases--when a "trilogy" is really a single large novel broken up into pieces, for example. In other cases, they bring nothing that simple bundling couldn't achieve.
A classic example of a case where an omnibus edition is more than just marketting is Roger Zelazny's original Amber series, which is really a single novel but got broken into novelette-sized portions or publication. With most volumes, Zelazny didn't even bother with cliffhangers or fake ending; he (or the editor) just reached a good break point and... stopped. The next volume picked up right after that with no recap or anything.
More commonly, the fake ending and recaps do exist and they do get in the way of the narrative even in an omnibus, which is why most are just marketting.
They usually make for good pricing but, in the ebook era, bundling or just plain discounting might be a wiser choice for all parties, unless the omnibus actually includes new material, like a framing sequence.
The omnibus edition of THREE TO DORSAI comes to mind there: it bundles three volumes of the DORSAI saga within a framing sequence that happens during a latter volume. But those kind of moves are rare; usually publishers just cut-n-paste two or three texts into one file and send it out under a new title as a "new" book. A good way to move backlist titles without explicit discounting or undercutting the "Perceived value" of books.
Again, mostly marketting.