At management level, you may only see those complaints which are escalated. It is probable that the front-line people receive and often resolve many more similar problems on first contact. Whist this is good, you are missing out on all the priceless feedback your customers are giving you for free.
Unless a proper system is in place to track and manage all complaints, you won’t know that the same issue comes up time and again. That also means you don’t have an opportunity to change something to prevent it from happening again.
Without a proper system, front-line people don’t have a way to capture information in a consistent manner. When they do anything, it is in a non-structured method which cannot be analysed.