Sometimes you want to copy a set of files from Dired in Emacs. If you have one or two files you can simply mark and copy them but if there are many files it can take some time for the copying and during that time Emacs is frozen.
The excellent abo-abo over at (or Emacs posted a nice bit of Elisp that arranges for the copying to be done asynchronously so that you can continue working while the copying takes place. The idea is that the code calls rsync asynchronously with async-shell-command and then switches to another window with the progress bar so that you can check the progress when you like while doing other things until the copy completes.
Since the copying uses rsync the code works nicely with remote copies too. If you often copy many files at a time, you should consider integrating the code into your workflow.