One of the PRIMARY purposes of threat intelligence is to understand:

Threat intelligence is evidence-based knowledge about existing or emerging threats that can be used to inform decisions. Its primary purpose is to provide context about the threat landscape, including who the adversaries are, their capabilities, and their intentions. By integrating this external intelligence with an internal understanding of vulnerabilities, an organization can more accurately assess the likelihood of a breach. This allows for a proactive, risk-based approach to security, focusing resources on the most probable and impactful threats.

A. Understanding zero-day threats is a specific, albeit important, component of threat intelligence, not its overarching primary purpose.
C. Understanding asset vulnerabilities is the purpose of a vulnerability assessment; threat intelligence helps prioritize which vulnerabilities to fix based on threat actor activity.

1. ISACA. (2015). Responding to Targeted Cyberattacks. Page 11. Defines threat intelligence and explains its role in understanding threat actors and their methods to better prepare for and respond to attacks.

2. ISACA, IT Risk Fundamentals Study Guide, 2nd Edition. Chapter 2, "IT Risk Identification," discusses threat intelligence as a key input for understanding the threat landscape and improving risk identification.

3. Patel, A. (2019). Cyber Threat Intelligence. In: Cybersecurity. Apress, Berkeley, CA. https://doi.org/10.1007/978-1-4842-5232-010. This chapter explains that a key outcome of threat intelligence is to provide the context needed to assess the probability of an attack.