While a lot of folks (especially the GTD-oriented ones) don't like to prioritise their tasks, many others do. The check box is a standard check box, meaning "done", but the little blank space next to it is for setting a priority (and perhaps order, such as A3 or C2) if you integrate such things into your productivity methods. The goal here is flexability, and hopefully it should be a distraction if you don't need it.

all my best,
dj