If you're anything like me you like to do as much navigating from the keyboard as possible. I prefer keeping my fingers in one place and avoiding any unnecessary mouse-ing.
That being said, default buttons in dialog boxes can be both a blessing and a curse. Just about everyone has had a window pop up at some point asking a question or giving a warning. Those windows generally give you a couple options to choose from, often "OK" or "Cancel", and most of the time one of those options is highlighted depending on what your computer thinks you ought to do. For example, I'm using iTunes and I want to delete a terrible song (let's say it's something by 50 cent). I select the song and hit "delete", which brings up this dialog box:
At this point I can either mouse over to the box and click my preference, or simply hit enter and the computer will select the default option, in this case "Keep Files".
That's pretty convenient if you do want to keep those files, but what if you don't and you don't want to use your mouse?
Try hitting command and the first letter of the non-default option. In this case that would be "command + m" for "Move to Trash". The computer reads my keystroke as the equivalent of a mouse click on that button. The same thing works with "command + c" for "Cancel".
This doesn't work in all applications, but it does in iTunes and a lot of others.