Indeed it is long winded, and that's rather a lifesaver. If something goes wrong during a render, and your computer crashes (or whatever) at frame 300/500, well, you don't have to worry about re-rendering the first 300 frames; they're already done. You only need to do the last 200.

If you're rendering an .avi, or something that "outputs" directly into a playable movie, and your computer crashes, you have nothing.

And while it get's a bit off topic, it helps with render layers as well; if you have a large scene, you can divide all the scene's contents into different render layers, and composite those. If you ever dislike something about a particular object/particle effect, you can make your tweaks, and only re-render THAT particular layer.

So aye, it is another "layer" of work, but it is handy...