I often use this when trying to tweak cloud settings. Since they tend to render slowly anyway, you can let it render a patch of cloud and then change a setting to see the effect of the new setting next to it. It's quite useful, because you can't always judge the effect of your changes with low quality renders, and the high quality ones can take too long.
In one of the earlier versions I also experienced the odd behaviour of starting a render and then opening the same file in a second copy of TG only to have the settings changes applied to the render. Since then I always make a copy when working on alterations while a render is in progress. Haven't tried this with the latest version.