Hi and thank you for your question. I will be happy to assist you.
It is important to handle all customer complaints because most, if not all businesses are driven by customers. One of the driving factors of getting more customers is word-of-mouth. If you have a customer that has an issue and you don't take care of them/resolve their issue, they are more than likely going to tell all their friends that your business is subpar. They in turn tell their friends and this reduces your customer basis.
A customer that is well taken care of especially when they have complaints will provide you with free advertising essentially. Even though they had an issue, you were able to resolve it so that keeps them happy and they will tell their friends how you have helped them.
I hope this helps, if you need further assistance please let me know!