Upgrading the clan castle castle can help you in many ways, but its function which is the most helpful to you is the housing of the reinforcement troops inside the castle.
Upgrading the clan castle each time will give way for a larger housing capacity for reinforcement troops.
This means that you can have more troops hiding in your clan castle than can defend your village during raids or having more troops to deploy when attacking.
The maximum clan castle level has 30 housing space for reinforcement tro
Other things that would happen when you upgrade your clan castle is increasing its loot capacity.
Clan war bonuses are stored directly in in the clan castle.
The more upgraded the clan castle is, the more loot it can store for you.