There are some cases where certain files open in applications not well suited for the current task at hand.
Personally, this happens to me when I create JPGs in Photoshop. Mac OS X naturally wants to open it up in Photoshop. Surely Photoshop can open it (it did create it after all), but for preview purposes it can be a slow process. Opening the file in Preview, on the other hand, can offer a much quicker solution.
Enter DefaultApp, a preference pane that makes system wide default app changes. You no longer have to set the preferred application for each individual file you create. Just set it up once, and forget about ever setting it up. It’s that simple.