Why do writers offer to run their own work through copyscape?

Something I’ve never been able to figure out, maybe someone else out there knows:

Why would a writer run their own article through copyscape to be sure that it’s original?

This makes absolutely no sense. . . unless, of course, they can’t write.

The only thing that I can think of is if the “writer” is more of a “sentence jumbler” and really copies things and adds a few adjectives to it. If this is the case – these people shouldn’t be “writing” in the first place. . . maybe they can handle a job as an article re-writer, but if you ever come across a writer that checks their own work using copyscape to be sure that it’s original. . . I would stay clear.

Unless I’m missing something? Let me know if I’m wrong.