The general philosophy with OF seems to be that tasks are things that can be accomplished in one sitting. You don't have to use it that way, but that way of thinking helps me understand how to best work with the tool.
With that in mind, "start by" and "due by" can be seen as essentially the same thing for a single task. On the other hand, for a project with multiple tasks, it makes a lot of sense to have a "start by" date to get the ball rolling on the project. For that, I tend to put a due date on the first task of the project. Often that task will be something like "Do some back-of-the-envelope planning for project Foo".
Hope that helps.