The wort grant serves two main purposes.Depending on the size of the system it may serve a third purpose as well.  

The first is to break the vacuum that can create on the grain bed from the pump pulling the wort out of the lauter tun.  

This can cause the grain to clog the false bottom and "stick" the mash.  

So the wort drains from the mash using only gravity into the grant.  

The pump then pulls from the grant, which has an open top or an air vent, and no vacuum can be created.
The second main purpose is to see the wort which allows you to determine how well it has clarified or when your sparge is coming to an end.
The third depends on the size of the system.Some larger systems have multiple drains coming out of the mash.

So with a large grant you can determine if one of them plugs or is not flowing properly.