Why should I fill my dragons with the maximum troops?

Because troop volume is your fuel AND your punch at the same time. Every hit a dragon lands strips troops from the enemy; when a dragon reaches 0 troops it flees the fight. So the more you load:

• the harder the dragon hits (volume feeds the damage formula),
• the longer it soaks and stays across the 10 rounds.

Fights are often won by accumulated wear: a full army melts the enemy round after round while it never heals. A half-filled army runs dry before it's done the job.

Absolute rule: before every departure, check that your dragons are filled to the max. If you're short on troops, produce more at the Barracks first. Never leave with "half" armies.