1st we create base layout for sculpting then we sclupt whatever we want, then we retopo it, but now  i get confused because after making retopo of it we again apply multiresolation modifier in retopo model and sclupt our final detail and then we agan make retopo and bake our final details on it . 

1. why we sclupt it again with multiresolution modifier .?

2. should we need again retopo it on , [after multires. model ] final detailed sculpted model.?