Its not as unrealistic as you think. Databases for example do pretty much exactly this. If you don't need the database to survive a reboot (which is often the case with search indexers for example) then this would work better with the database on a ram-disk to avoid the random-access writes to disk after every database access.
If the best example possible is copying files, I believe my point has been proven as it again is a very edge case example - you don't normally copy files for no purpose (backing up might constitute the purpose if they weren't RAM drives).
If you don't like that example, take the example of storing Temporary Internet Files or the system TEMP directory on a ramdisk.
Or if you don't like that, how about disassembling a movie into lots of bitmaps to push to a program that modifies the bitmap, only to recompile the bitmaps back into a movie at the end?
Or Video streaming when the disk is in-use/old/slow/unreliable
Or a place to download torrents to and from avoid lots of seeking during the random-access read/writes
Or as a destination to unzip temporary files during an installation
Or to store your OBJ files during compilation
Or for disk-less computers where all data is stored on network drives