Fitts’s law states that things bigger and closer are easier to hit, as well as things that are at the edge of the screen.
Why at the edge?
Because your cursor has nowhere else to move. Its therefore impossible to overshoot past the corner or edge. However, with the prevalence of multi-screen desktop set-ups this becomes less relevant. Think about it, how often have you ever accidentally strayed into your second screen with your mouse?
Visualising Fitts’s law
Why’s it important?
… correct application becomes even more important for those with limited motor skills such as children, the elderly and the disabled. Targets need to be even larger to aid in the success of the user pointing at the target.