im new here..
so what are clans and how are they useful?

Clans are a groups in the game the owner of a clan can upgrade it after there is enough money in the clan vault. Clans have rewards which you can obtain by exchanging clan credits and money

Clan? Clan is like a group. Which they will help you when you are in need.( Only to clan with kind people, like mine lol! ).
Other then that, there is Clan Contest in some clan too. There is also Clan Reward..