Right, the vorlauf answer was regarding the question by stevefry.
As already mentioned, the purpose of a grant is to prevent the head at the bottom of the grain bed to get too low. If the difference between the head on the top and bottom of the bed is too large, it can cause the bed to compact and plug. Many of you have seen the 'gooseneck' outlets on historic mash tuns in large breweries. Those outlets are also a measure to help reduce that head difference imposed on the grain bed.
With a grant in place, a pump intake can be inserted there and it will just pump the grant dry instead of applying a suction on the bottom of the bed when a pump intake is connected directly to the tun.
A grant is not really needed. The pressure at the bottom of the grain bed can be regulated by throttling the tun outlet valve. Even if a pump is attached to the tun outlet (like with RIMS or HERMS), the pressure at the bottom of the bed can be reduced by closing the discharge valve on the pump to limit the flow rate.
Including a sightglass (manometer) that is ported to the bottom of the grain bed enables the brewer to avoid placing too much head difference on the bed. By monitoring the head in the sightglass, you will know when to throttle either the outflow valve or the pump.