For handling moderately sized chunks of data in which there is a lot of random access. Say, if you have a 200MB database that is killing your poor HDD.
As well, note than modern OSes (Windows 2000 and up, OS X, most Linux distros) will cache parts of the filesystem in RAM, and try to intelligently manage how much is loaded, to give good speed. They don't do this perfectly, because there are many 'gotcha' scenarios to avoid, but they do a pretty good job. So, overall, no, there aren't any novel uses for a RAM disk on a desktop.