But…the goal in aoe2 is NOT to “keep your army healthy”. It is to push the enemy off the map.
Many lower Elo players are way to hesitant to use their army. They think keeping it back and healing up is free. But it isn’t.
Try to think of your army as “reverse villagers”. Instead of creating ressources for you, an army takes away ressources from the enemy. If your units hit buildings the opponent repairs, this becomes really obvious: Every hit you do destroys a bit of wood/stone. But even if you use your army to pick of a few stray units or raid the enemy farming eco, you always destroy ressources.
Now, just like villager idle time is horrible, army idle time can lose you games. So garrisoning your army to heal up is NOT free. It comes at a huge cost, and very often is not worth it. Thats why herbal medicin is a niche tech: You need an expensive army, several castels, your army has to be damaged, and, most importantly: The game has to be in a state where your army can’t do too much damage anyway. That just doesn’t happen all too often.