You can drag a file onto an application icon to open it in that application. If the application is able to open the file, its icon is highlighted. Drag and drop is a good way to open a file in an application that isn't the preferred app, because it doesn't reset the preferred app. For example, let's say you want to edit an html document as text. Rather than double-click the icon, which would launch NetPositive, you can drop it onto StyledEdit, Be's simple text-editing application.
When you drag a file and hold it over an app that doesn't know how to open the file, the app's icon doesn't highlight, and the app won't open the file. To force the app to accept the file, hold down the Control key when you drop the file icon.