The MAIN purpose of a risk register is to:

The main purpose of a risk register is to enable well-informed risk management decisions by providing a comprehensive and up-to-date record of all the identified risks, their analysis, and their responses. A risk register is a tool that helps to document, monitor, and communicate the status and outcome of risk management activities. A risk register also facilitates the review and evaluation of the effectiveness of risk management processes and controls. Documenting the risk universe, promoting an understanding of risk,and identifying stakeholders are possible benefits of a risk register, but they are not the main purpose. References = Risk and Information Systems Control Study Manual, 7th Edition, Chapter 2, Section 2.1.1.3, page 531

1: ISACA Certified in Risk and Information Systems Control (CRISC®) Exam Guide, Answer to Question 640.