1.They provide useful buffs to clan members. We generally have them set to provide an xp bonus. Members who have activated their weekly buffs receive a 3% xp bonus for most skills when they are on the same world as an avatar rising to 6% if they are within 10 squares of the avatar's location. 

2. To activate weekly buff - After joining you will notice orts appearing on the ground. Pick them up and when you have 300, locate an avatar and interact with it. Note that there is a maximum daily amount of 200 that you can collect. 

3. Only Admin ranks and above can take clan avatars out.