1) there is no risk in giving cancelled cheque leaf
2) the reason why broker firm is asking for cancelled cheque leaf is to verify your bank account details
3) Know your customer (KYC) is very essential for a lot of investments these days. KYC is needed for mutual funds, stock investments, etc.and cancelled cheque is required by the firm for verification purposes
4) ECS - Electronic clearance service (ECS), which is used for deducting money from your accounts every month also require a cancelled cheque.
5) A demat account is needed for anyone who wants to trade or invest in stocks. A cancelled cheque needs to be submitted to the stock brokerage along with the account opening form, ID proof, address proof, etc.