in the dev streams, they said you can use the garrisons to reinforce an army in battle. while playing around, I noticed you're only able to do this if the "battle" happened while one side (ie. the enemy as defender) was besieging a province (ie. your province), as you could only press/access the sortie button using the siege view interface, and the siege view interface is only visible if there was a prior siege.
In short, you can't use your garrison to reinforce a battle if it happened when one army "catches" another army (that's moving, thus not sieging) over your provinces. will this be changed sometime in the future?
I've found that sometimes a 2,000~3,000 soldier reinforcement is often enough to win a critical close-call battle in your favour - especially if it's a very large battle that involves entire force limits of countries, and morale is at single digits for both sides (like 0.09 vs 0.06 - a single reinforcement unit or two can completely tip scales towards victory, especially if reinforcements are too far away )
In short, you can't use your garrison to reinforce a battle if it happened when one army "catches" another army (that's moving, thus not sieging) over your provinces. will this be changed sometime in the future?
I've found that sometimes a 2,000~3,000 soldier reinforcement is often enough to win a critical close-call battle in your favour - especially if it's a very large battle that involves entire force limits of countries, and morale is at single digits for both sides (like 0.09 vs 0.06 - a single reinforcement unit or two can completely tip scales towards victory, especially if reinforcements are too far away )