There are two reasons for people to donate, attacking and defending. Defending is the most popular. The most requested troop for the clan castle is Archers. Barbarians work the same. Here's why. With a full clan castle of archers (say 20-25 archers) the attack power is 400-500 hit points per second. That's higher than any defensive building. Each archer might have lower overall health than a giant, but with a clan castle full of giants you will only get 4 or 5 troops. If you get attacked by 50 archers or barbarians they all target one troops at a time. So with 50 troops against 1 giant they will demolish your clan castle troops within 4-5 seconds. If your clan castle is full or archers or barbarians it will increase this time to 20-25 seconds. With this extra time it gives your defensive building 4-5 times longer to eliminate the enemy troops. Keep this in mind next time you request or donate.