The number one reason to become certified is to help ensure patient safety. For the NP who is driven to be certified it is often to fulfill a personal need to be professionally recognized as having obtained a higher level of excellence in their practice. Putting "CNN-NP" next to your name shows your colleagues that you have the advanced knowledge and competence as a nephrology NP and you provide an excellence of care that is the desired by our standards of practice.
Some other benefits of becoming certified can depend in part on the support the NP gets from the practice they are employed by. Many practices pay for the certification exam, provide extra benefits or financial incentives and acknowledge the clinical excellence you can bring to their practice.