A couple of weeks ago, I set a goal to finish the first draft of my WiP by the end of the year. Well, that didn't happen. When I set the goal, I knew it wouldn't happen simply because I had so much going on in my life at the time, there was no way reach the end.
But I did get over 15k words written because I pushed SO hard! And that's why I set the goal. I needed something to strive for that, if given enough free time, I could manage. I'm absolutely certain that if I hadn't gotten sick on my vacation, and if I hadn't taken on other responsibilities throughout December, I would be ready to revise my novel.
Without that goal in mind, I had the motivation to create scenes that excite me, push the story forward, and give me something to work with when it's time to improve the book on the second pass. The scenes, as they stand, aren't the best I've ever written, but that's because they need some serious polish. But they have potential. And I fought my way through them with the enthusiasm and gusto of someone working on a serious deadline.
As I mentioned, I did get sick over my week off. The week I was supposed to spend with my wife doing fun stuff around town, and I honestly wanted to get more writing done. But since both of us were sick, we spent most of the week sleeping and watching TV. As a professional, I will work when I'm sick. I've worked ten hour days with a fever of 102 before, and while it wasn't fun, my over-developed work-ethic won't let me simply back out on my obligations. When I'm writing full time, that will be true for deadlines that I have to make with my work.
In summation, goals are important for us, even if we don't reach them as quickly or efficiently as we hope. As long as they motivate us.