I've been asked on many occations an interesting question. Why, if I've finished the rough draft, or even the final draft, on a book that's part of a series, especially a long one, why do I wait to publish it until the rough draft on the next novel is complete? Well, there's reasons for that. The first reason is simple. It keeps you from writing yourself into a corner. How so? Well, as any good author knows, you don't write the story. The story writes itself. You're just the scribe. And even though we do have some say in where the story is going, ultimately it chooses what tale it will tell. And if you're not careful, that great idea you had in book 2 may backfire royally on you in book 3.
I learned that lesson the hard way early on in my writing career. Take the Earthfleet saga for example. That's got a bunch of really messy transitions and plot holes in it that, thankfully enough, most people didn't catch. But me, being the author, they shine out at me like a spotlite. For example, there were several major character and race introductions that ended up not being used anywhere else in the series, and eventually were completely dropped from the cannon, even though I'd spent chapters developing them. If I were to properly fix that, it'd require some serious rewriting. That could, of course, have been avoided had I learned and followed that rule early on.
The second reason is simple. Even though I'm the author, sometimes even I don't have any idea where the story is going, and that can sometimes cause issues in planning how the story will unfold. I can't tell you how many times I've written down this great and amazing ideas and plotted out the complete storyline only to have the actual story take things in a direction I never anticipated, leaving me with a huge problem further on down the series. So, rather than playing an epic version of Twiter to try and fix the massive hole I've gone and dug for myself, writing ahead and completing the next book before finishing the prior one helps avoid that. Now, it won't stop every problem, but it does eliminate a lot of them.
Really, the only way to prevent any plot holes and problems, especially on a long series, is to write the whole thing and finalize everything before publishing, and who wants to do that? Also, writing ahead helps you better picture where the story is going so that you can fix any issues that might appear in the soon to be published story before it goes out. Not major plot holes, but rather simple story flow and mechanics issues. For example, you're not going to have a character be all prim and proper in one book only to have them become light hearted and giddy in the next. That might sound like a plot hole, but really it's about mechanics and flow. Of course, if taken far enough it can easily become a plot hole. But in general, something like that falls under the idea of story mechanics.
Anyhow, I hope that explains it a little better. If you have any questions, please feel free to ask. :)