Quite often, I see posts asking why writers write and the answer usually comes back with "I JUST HAVE TO! I HAVE TO CREATE!" While I understand that urge--I have it myself--that's not true, at least not all of the time.
First, who is a writer? Do you write school reports? Blog posts? Books or stories? Tech reports? Anything at all? Congrats, you're a writer!
From the examples listed above, you can tell that I consider anyone who writes a writer, even if they have to do it for an assignment or job. While some jobs or reports may tap into creativity, more often than not they require discipline to complete, especially as a person may not be especially inspired to write a paper on what mitochondria is and what it does for the body. (But they might be; I'm not judging.)
So, let's narrow it down to writers who write fiction. Many times, writers will answer this question with "I just gotta create!" and that's great. However, there are a lot of people who choose not to write their fiction stories down and simply dream of writing the great American novel. Thus, discipline is needed to sit down and actually write. The creative impulse is important to get the whole process going, but... it's discipline which enables you to actually get it done. On the other hand, there are writers out there who write by the numbers books to just get a paycheck. Yes, there's still the creative spark present, but it's more rote, with less drive other than "I wanna get paid".
Hence, I think both are important. Without the spark of creativity, your stories will probably seem like anything else in its genre without anything really new to add. Yet without discipline, you'll never get the story out. The answer for me for the "Why do you write?" question is pretty easy: I want to and I have the time to.
Just random musings while I'm waiting to go home from work, heh.