hope below helps!!!!

COW sites serve two purpose

+ Todays bts cost has come down considerabley.Earlier the cost were high.Also the cost of bringing up the bts infra was high.Pls remember earlier infra sharing concepts was not introduced initially.On a vergin mkt where you are not sure of the mkt base or just to ensure you have a coverage...COWs were introduced,so that incase later it is found that if the cell site load is high or economically viable,you can covert it into proper bts cell sites.

+ secondly as others said it is also used to give coverage in area for specific festival where normally you wont see high calls or just to add capacity.