Having put the PhD into the binders, I've gone back to the novel I was working on before academic stuff took priority. It's an intriguing experience going back to things after a short break. You come at it with fresh eyes, and so very quickly get a feeling for whether what's there is any good or not. I'm happy to say that I liked what I'd already written, and quickly got on with writing the next bit.
I think that's the other main thing you get from breaks: a burst of enthusiasm and ideas afterwards. In fact, in creative contexts other than writing, it's considered fairly normal advice that you should take a break now and then, so that you can come back with new inspiration. Of course, that runs counter to the normal writing idea of writing every day to get into the habit of being creative, so which approach works best?
I suspect that, as a general thing, it's better to write than not. Stopping every time you don't particularly feel like writing anything that day can easily turn into "I haven't written anything for weeks" without you realising. Writing something each day is probably a good base point. Probably better still is wanting to write something each day. After all, we do this because we enjoy it, don't we?
At the same time, there are times when it's easy to get stuck in a rut. You've written essentially the same story a few times in a row. You've repeated ideas throughout the novel. You can't tell the last half dozen poems apart. That sort of thing. Or worse, whatever your creative endeavour, it's starting to feel like Work. I'm pretty sure that's the oposite of the sort of thing we normally want.
So getting away from things is sometimes a good idea. The trick is to judge when you need it, and I suspect that comes back to the idea I mentioned above, of wanting to write every day. Ultimately, most of us aren't paid for this. Or at least, we aren't getting rich from it. We write because we want to. I think that the key to knowing when to back off is paying attention to whether you're enjoying it. If you have a day when things aren't going well, then fine, you might want to push through it. If you have a string of days when you just hit a blank, or when it feels more like work than fun, then it might be time to have a break.
I once went through a phase of trying to practise the guitar properly every day. I'd do my warm ups and my scales, and try to move up a notch on the metronome. I'd do the same exercises over and over, and pretty soon I found that I wasn't looking forward to playing. Now, I play more or less when I feel like it. The thing is, I probably play more now than I did then, and my playing is better for it. Sometimes, if you focus on simply enjoying what you're doing, you get far more done than when you try to force it.