When you want to get a screenshot if you stop it before it's finished you lose all the part of the image which had already been rendered. But sometimes you are rendering from a very high point (maybe because you can't get closer in bird's view) and you are only interested in this central part, the one which has already been done.
So.. what about keeping this if you stop the render, instead of losing all of it? I mean, you will have some black parts at the image after stoping it at the middle of the job, but in the end you get rendered the part you are interested in (probably you will use some image-related app such as "the gimp" to cut the important part of the image).
This could be even more useful for video.
From version 1.8 of the Photo-video rendering plug-in, it is possible to save an image after a rendering has been stopped.