It also prevents using Ctrl + A to select all the content to copy and paste. Someone could still copy text from your source code, but this will stop some and slow down and make it less convenient for others. It won't work if the visitor has JavaScript disabled or has a browser that doesn't parse JavaScript, but that isn't very many.

Most content thieves are lazy, that's why they don't write their own content. Using this with a no right-click script will send many of the would-be thieves looking for an easier mark.