Fundamentally, where Coda shines is as an IDE for online development. It works for offline development too, but it really shines if you have access to your server (which can be on your dev box). It lets you write and debug code interactively, even if the code is on a server on the far side of the planet.
Under the hood, Coda is essentially a cut-down version of Transmit FTP integrated with a very good, simple text editor, web browser, css editor, and online reference. That's it, but the combination is so slick that the results are magical.