I keep almost everything I've ever written. Seriously! I have an enormous box of journals dating back to the 1970s and decades worth of computer files that are so old I probably can't even access them anymore. (Yes, my teenage son does bust on me for this!)
I have my reasons, which can be boiled down to these.
1. It's work I've created.
2. It can serve as inspiration for future work.
3. Jumping into a piece or project that's already started is far easier than starting from scratch.
So, as you might guess, I have a lot of stuff to manage. But when I talk about decluttering for writers, I'm NOT talking about tossing old notebooks and journals. (Keep those!) I'm talking about clearing out other clutter - the regular kind that gets in your way on a daily basis and belongs in the trash or donation bin.
I don't know why exactly, but decluttering physical stuff helps me clear brain space, so I can focus on writing. I can't show you pictures of the inner workings of my brain, but take a look at these "before" and "after" pics of my attic. I can hardly wait till the next time I have to go in there for wrapping paper!
More on this topic on my podcast: Better Life, Better Writing!