DDB also adds a ton of value to the product for free. Moreso than iTunes does, by a wide margin. iTunes is fine, but it’s much less useful than Spotify, IMO.
Literally the only difference is the amount of work involved in adding your already purchased content to the service. Ripping a CD is easy, inputting the book data is...well also extremely easy, but more time consuming.
Well, that, and the fact that DDB is incredibly useful, and comes with a character builder, and makes all your game data into a single integrated service that cross references, with things like hyperlinks and tool tips, so you’re also getting an enormous increase in utility over the alternatives.