To form a habit. It's advised that writer learn to write every day. Put time aside, hands on keyboard, and write something. However, what if you established the habit. It's part of your daily life. Do you need to write every day?
My answer is no.
Learning when to take breaks is important. It keeps your writing process healthy. Most of us out there aren't the romantic notion of what a writer is anyway, so there is no need to suffer for your art. Well...some suffering. But not easy, preventable suffering.
Taking a break really helps prevent burnout and fatigue. Which is something I'm very familiar with. You don't want burnout. It isn't fun. It's frustrating. So now I make sure I keep my mind fresh. And to do that, you need to refresh it with a good break.
So how often should you take breaks when writing? Well, it depends. Sometimes just walking away from something and do anything else helps. My favorites are just sleep on it and set aside an entire day of no writing. A rest day, if you will.
And remember, taking a break is not a sign of weakness or laziness. In the past, I've seen writers in writing communities look down on breaks or suggest you need just short breaks. You can just plow through your issues. Sometimes you can, but for other moments you just need a break.
Also, something to keep in mind is not let taking a break be an excuse for procrastination. And that is an entirely different beast.