I attribute the application of this idea to what was then called Computer Security to my mentors, later colleagues, Robert H. Courtney, Jr. and Robert V. Jacobson. They did it in an attempt to rationalize decision making, more specifically the allocation of scarce security resources, to the then nascent field. They did it in response to their observation that many, not to say most, security decisions were being made based upon the intuition of the decision maker and their belief, and a tenet of this blog, that security is a space in which intuition does not serve us well. They wanted to bring a little reason to the process.
They could not possibly have known that in a mere fifty years that the resources applied to this effort would grow to the tens to hundreds of billions of dollars, that the safety and liberty of the individual, the health of public and private enterprise, the efficiency and resilience of our economy, and the security of the nations would turn on how effectively and efficiently we used those resources.
So, at its core risk assessment is a decision making tool. It is a tool that we use to answer the question "where to spend the next dollar of our limited resources?" Courtney's Second Law says one should "Never spend more mitigating a risk than tolerating it will cost you." We will, do, make this decision, with or without tools. We make it intuitively or we make it rationally but we do make it.
At its most elaborate risk assessment is a very expensive tool requiring significant knowledge, skill, ability, and experience to use, more than most of us enjoy. It should be used only for expensive decisions, decisions that are expensive to reverse if we get them wrong. At its simplest, it protects us from making decisions based solely upon the threat, attack, vulnerability, or consequence de jour. It protects us from intuition, from fear.
All that said, few of us are confronting expensive or difficult decisions, decisions requiring sophisticated decision making tools, risk assessment or otherwise.. We have yet to implement all those measures that we know to be so effective and efficient as to require no further justification. They are what Peter Tippett calls essential practices. Anyone can do them, with available resources, they are about 0.8 effective but work synergistically to achieve an arbitrary level of security. They fall in that category that we call "no brainers." All we need is the will.