There is no such thing as wasted time unless you are wasting time doing something you do not enjoy or it is something you do not gain anything beneficial from.  Going to work may not be enjoyable, but the paycheck is why we continue to go.  As such, though we would rather be doing something else, the time is not wasted.

But what about when we write?  Do we only need to write things that we will use?  Is there such a thing as wasted writing?  The answer of course is a resounding NO!  There is no such thing as wasted writing.  Every single time you write, you are practicing.  That in and of itself is reason enough to participate in throw away writing, but there are many more reasons, a few of which the Demented Elves will share.  

First off, throw away writing is helpful when writer's block has set in.  When there is no pressure to write anything of value, the brain is free to force the hand to write down what it wants you to write, thus dislodging the blockage you may be experiencing.  This too is reason enough to take up throw away writing. but there are other reasons as well. 

Another example would be, if you do throw away writing on days you are not working on your projects, you will keep the habit of daily writing.  This is important.  Any writer or writing instructor worth their salt will tell you daily writing is essential.  Demented Elves contend that any writing will do, just as long as you are writing.  Even things such as this blog post counts as daily writing.  Remember the more writing you do, the better you get.  The better you get at writing, the better a story teller you become.  The better story teller you become, the better chance you have of selling your story.  And we could go on and on and on and on.... but the Demented Elves are certain you can come up with your own reasons to participate in throw away writing.  We think you get the idea.

One parting thought:  All those reasons are good reasons, but my favorite reason for doing throw away writing is... IT MAY BE SOMETHING YOU WISH TO KEEP IN THE END!