The Clan Capital is your alliance's main stronghold on the World Map and is unquestionably one of the most important structures in the game.
The Capital allows your Clan to claim territory on the Kingdom Map. It also allows you to:

1. Build more Clan buildings.
2. Send caravans of resources to Clan buildings.
3. Defend against enemy portals, since they can't be opened on Clan territory.
4. Benefit from Clan bonuses.
5. Kick out unwanted players from the Clan's territory.