i don't have any player with this SA and i'm thinking to buy someone the next season.. but i really don't how it's work.. i mean, what is the bonus in fact?

Should i buy a DC/DR/DL for defend the corners of my rivals?? Or any tipe of MC to shoots my corners?? Or ST to try to get goals of my corners??

Do really this SA worth to look and buy a player with it?

PD: sory for my english