How important is customer retention?

Very important! Attracting new customers are not as efficient as returning them. A loyal customer will always return to your shop to choose your products or service, and you are creating a relationship with them, so the trust they have in you will be stronger, and they will listen to your recommendations and suggestions. And they will probably talk right about you to their friends, at their job, etc. Investing in your actual customers is not easy, but it's worth it. Without a strong client base, you would have to rely on constant marketing and ads to keep attracting new ones.