Routine - it's something we all need in our lives in some way, shape or form. For a writer, its essential to keeping his/her skills sharp. As with anything, the more you do it, the better you are at it, like playing the piano or competing in sports.
I always love the stories at writer's conferences, and Nora Robert's comments always surface about when she's writing and her kids used to call for her. Her response was that unless the house was on fire or someone was dying, writing was HER time and she shouldn't be interrupted. That reflects her dedication to her craft and certainly she has a routine.
When I first started writing seriously, and I hear this from many people as the way they start, my kids were young and my husband worked a third shift job. So from 8:00 pm on, I had the run of the house to myself, and my love for writing kicked in with the free time. The kids grew, the husband changed and routine changed, but the writing remained. The important part was that it remained. I continued to make time for it and to fine-tune it. The difference is in altering the routine - the when part of writing.
We all live by a circadian rhythm. We go to bed at a certain time, we wake up at a certain time. Our pets expect to be fed at a certain time. These are all routines that we live our lives by. Even the homeless on the streets find their haunts at certain times of the day.
While there's something to be said for spontaneity, routine is inescapable. And as a writer, it is essential.