Great Journey is hard to predict as it can be used in various situations. Offensively, it allows the player's army to reach the enemy's town sooner, limiting the time they have to train units or heal their existing ones. The player's army is also more capable of picking out targets in battle and can keep up with the enemy should they try to escape.
Alternatively, it can be used to escape a situation where the odds are not in the player's favor or retaliate against an attack by bringing home the army faster. Should the player's Peasants be raided, it can also be cast to help them to run away from harm. Siege weapons benefit the most from Great Journey as they are otherwise very slow.
Another, albeit uncommon, use for Great Journey is to boost the economy. All the player's units without exception are affected by it, including Peasants which will take less time to walk between resources and drop sites. It also enables Caravans to travel between the Town Center and Market at a remarkable speed, increasing gold income.