I've used the Feedback module for a very long time, but I've never needed to use the feedback block. However, one thing you can use it for (and I think the only thing) is if you add a feedback activity on the front page of the site (course 1), then you have an option of Mapping that feedback activity to any other course on your site. Look at the options for a feedback on the frontpage and you will see the Mapping button...see below. If you Map to courses, then when you add the feedback block to those courses, it just displays a link to the feedback. To my knowledge, there is really no reason to show a feedback block on the frontpage of the site. This could be handy if you wanted to create a common feedback for several courses...just add it to the frontpage and then map it to the courses and show the block in those courses.
Note: This is on a 1.6 install...I don't know if anything is different on later versions.