Difficult terrains don't only slow your troops down on the strategy map, but also on the battle map. This can make it difficult for non-flying melee units to close with their opponents. There is no skill that solves this problem (the Pathfinding skill only works on the strategic map), but this spell does. Use it to help your heroes and melee creatures close more quickly with those pesky ranged attackers over disagreeable terrain. Overall, use this spell only if you don't have anything better to cast.