Sending a legal notice is in some cases a mandatory step before initiation of legal proceedings. For example if you have a claim against a cheque that was dishonoured then not sending a legal notice within 30 days of it getting the cheque returned from the bank prohibits you from proceeding under section 138 NI Act which provides a fast and easy remedy in such a case. Then you are left with the option to file a civil suit for recovery of payment/ money which would be a lengthier process often times taking a couple of years for redressal.