You know, it isn't just about writing 'something' every single day. It's much deeper than that. While some may find it useful to just free write about any old thing, hit a target word count, and call it a day, I think most people who are trying to write every day are trying to write something specific. Maybe it is about journaling, or maybe you're a freelancer who's putting out articles.
For me, it is about telling stories. It's about learning how to be a better storyteller. The writing that I'm doing every day (or almost every day) is about producing some finished piece. Right now, it's about finishing a novel for younger readers, that blurry line between children's book and young adult book.
In other words, there has to be a focus for the daily writing to continue. Writing for writing's sake isn't enough. For me, while writing certainly about self-expression, it is also about conveying a message to somebody else. Even when I'm handwriting in my journal, it seems that my prior freelance writing experience, spills over and I'm conveying what I think, feel, and desire, but in a way that seems as though I'm talking to somebody else. These aspects go hand in hand.
There is another sort of focus I have in mind as well. One thing you will realize is that if you don't have something you're trying to improve in your writing, especially as a fiction writer where the concept is telling a good story, you might end up spinning you're wheels and not making any progress.
Think about all of the books you've read and really enjoyed. The masters of storytelling captured something in their writing that grabbed you and held you there, captive until the last word. What was it? It could have been some aspect of the story structure, or compelling characterization, or maybe intense cliffhangers at the end of every chapter that kept propelling you forward because you just had to see what was going to happen next.
The craft of storytelling is a learned skill and if you want to improve in it, you will have to study other writers, learn from them, and implement what you learn into your own writing. If you write fiction every day, ask yourself this question: What aspect of craft am I going to focus on with this story or in this chapter?
For me, I'm trying to use cliffhangers at the end of chapters or at least leave things unresolved enough to prompt the reader to ask, "what's going to happen next?" and maybe keep reading the book.
There will always be something to learn about writing and ample reasons to practice. I would add that the more you practice, the better you will get, if you continue to learn something new about storytelling. Bestselling writer Dean Wesley Smith speaks a lot about the importance of focused practice and writing more to become a better storyteller. For him it is crucial. I want to be a better storyteller, so I will write more. It's as simple as that.