I find that when I write, I’ll often come up with phrases, sentences, and images which I really like a lot, but which don’t fit at all with what I’m writing. Likewise, I find that sometimes I’ve written something that’s total crap, or that just doesn’t work for some reason. In both cases, I get rid of the things that aren’t appropriate to the piece that I’m writing, but I don’t just delete those things, especially in the first case where I’m enamored of something that I’ve written.
The only instance when you have wasted your time writing something is that case in which you delete what you’ve written. For this reason, I try not to get rid of anything that I’ve written, even if that thing doesn’t fit at the moment. Instead, I have lots of plain-text files with scraps belonging to different pieces. Often, I don’t revisit these things because they’re just crap, but sometimes there’s something worthwhile buried in all those lines of text, and I don’t want to risk deleting something useful.
In this age of single-spindle terabyte hard drives, it’s not a big deal to keep around a couple dozen text files with unused writing, especially since plain-text files are tiny. A page of text, composed of two abandoned beginnings to one of my stories takes up five kilobytes. A few notes about a story idea takes up maybe one whole kilobyte. It would take one thousand such files to fill a megabyte on your hard drive.
In a similar vein, I’m an advocate of keeping previous drafts of things that you write. I’ve found in the past that, in the course of several drafts, I’ve taken things out which really didn’t need to be taken out, and without hard-copies of previous drafts of a piece, those things would be lost.
Lesson? Keep everything that you write, even if you don’t like it. At worst, it will sit around taking up almost no space. At best, you’ll find that you’ve written something that comes in useful later.