Perhaps the most important is that you'll gain new insights into what is important to your customers. This allows you to understand their priorities and buying patterns. Being able to constantly update your customer insight gives you a better idea as to how you can better serve them.
You may also come to find that receiving a complaint enables you to assess and put into practise your conflict resolution skills. It shows if you're well equipped to make proper decisions that help benefit both you and your customer. An immediate and quick resolve to any complaint increases your customers trust, leading them to continue doing business with you. It's a win for everyone.
This is an amazing situation for you to take a step back and reevaluate if your promises are being delivered. This reevaluation allows you to master delivering what you promised and exceed the expectations of your customers.
Another great thing you can learn is your flaws. Again, it might not be an easy pill to swallow but it is one that you can't afford to miss. Knowing your flaws allows you to improve your business plan enabling you to provide better services at a greater quality and at a good price.
There is a lot of invaluable information that you can gather from listening and dissecting a customer complaint. Keep an open mind and always be prepared to learn and improve. You'll find that in doing so you're improving your services while simultaneously growing your customers.
Gain new insight to what is important to customers
Enables the assessment and practise of conflict resolution
Gives the opportunity to reevaluate your promises to customers
Allows you to see your business flaws