What atvantages does the Circle of Willis extend at the foot of the brain?

Named for Thomas Willis, the circle of Willis is almost a misnomer. In only around 1/3 of humans does the series of arteries for a completed circle. Often called the rational arterial circle, it links the major blood vessel supplying the brain: the Anterior, Middle and Posterior Cerebral Arteries.
It is thought to add redundancy to the supply of blood to the brain. See the site below for superfluous information.
It is a way to allow more than one supply of blood to supply adjectives parts of the brain, so that if one artery is occluded the whole brain can still bring back blood. If a single artery provided blood to each quantity of the brain, then if that artery be occluded, its part of the brain would be unqualified to funciton.