Why is the role of secrecy very important in banking profession?

Because you are protecting the client and is one of the most important conditions in the contract between you and the client

In Banking Professional secrecy is the most important thing, since customer is keeping his money in bank on trust.If customer's financial matters disclosed then customer will not trust on banks.Breaching of customer's information can leads to serious harm for the customer.Thats why banks cannot disclose any information of customer until any legal obligation imposed.