Sand is added to football fields to allow the turf manager better control over the moisture management and so that the soil of the turf is able to resist soil compaction.
The grass on the football fields can easily be grown in the sand because it's highly maintained and well taken care of to allow it to properly grow in the sand.
The sand also helps give the football players traction when playing football so they do not slip and slide as much as they would on grass without the sand.
But mostly the sand is for the moisture management and prevention of soil compaction.