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It's useful to have Apps on the dock because you can open files intelligently by dropping them on the icon. Fo instance, if you drop a photo onto Mail a new message will open with the photo attached, by default a double click will open it in Preview but if you need Photoshop to make fancy edits drop it on that. For text files you might want the icons for Text Edit, Word, Dashcode, Text Wrangler. Much quicker than doing a right click and Open with...