Why is product loyalty important to product developers?

I assume you are asking a straight forward question and not the reversal (ie, you are asking "why is product loyalty important to product developers" and not "how does product developer improve product loyalty.")

Product Loyalty is important because it creates a "constrain" on how much to change. Ie, if you have a lot of loyal customers using a product and you are going to make a new product, you don't want to create a completely new design, because that will cause your existing loyal customers to go crazy. So, you want to make it easy for your existing loyal customers to use the new product yet at the same time, introduce new features/ways to use it.

Loyal customers are also more "tolerant" towards faults in the new product. They are more kind to "over-see" a minor design error as compared to a new competitor. "Resistant to change" is important key.