A wort grant allows you to drain the mash tun by gravity as opposed to directly pumping out of it.  This means less risk of a stuck mash caused by creating suction on the grain bed.  A wort grant is not necessary when using controls with peristaltic pumps. 

It will help increase your mash efficiency by allowing you to gently flow the wort from your mash tun, and not pull too hard on the grain bed.