When you copy multiple files in OS X and the destination folder already has files of the same name, you don't have a lot of choice over what to do — you can either cancel the copy or replace the files. This little trick gives you a bit more power.
It may not be as easy as Windows 8's new copy dialog, but OS X actually has a few hidden options if, say, you want to keep some of the old files and replace other ones. Web site Mac OS X Hints explains:
In Lion, one of the new features is to be able to 'Keep Both Files' when copying multiple files in Finder. While this is useful there are many times when the old 'Skip' behaviour is better suited to the task. Well, the (now) obvious way to do this is to just hold the 'Option' key, and then 'Keep Both File' changes to 'Skip.'
The "press Option for more options" rule has been around for awhile on OS X, but this particular tweak is Lion-only. It's tricks like these that make new versions of an OS great, even if the big features aren't all that spectacular.
Change the Finder Option When Copying Multiple Files [Mac OS X Hints]