It's easy to delete an 'in progress' file when you think you've finished a photo. I mean, it takes up additional space, right?
But as space is so cheap nowadays though, that it's worth keeping the in progress file for potential future editing.
As your skills improve, you may find you want to re-edit a photo, crop it or make a thumbnail from it, just as a few examples.
Without an in progress file, you have very little in the way of options.