Testing, in its core, is the act of figuring out things you didn’t know.
A test is to ask yourself a question, and then to perform actions designed to find the answer to that question.
The answers will tell you whether the product is ready for release – but only if you asked the right questions in the first place.
So how do you know whether you are asking the right questions?
Risk based testing is a methodical way to find the relevant questions, to prioritize them, and to challenge your beliefs and intuitions by exposing them both to yourself and to others.
An experienced tester will always have an intuitive sense of the relevant risks and priorities while testing, even without a formal identification process. For a small project, that might be enough. However, even the experienced tester is prone to cognitive bias. Making yourself think through a topic by making an explicit list of risks is a good way to overcome bias. Inviting others to join in the process is even better.
For a larger project, a formal risk analysis process is vital. On one hand, it is your tool to increase the likelihood that you will be asking the right questions. You know you won’t have time to test everything, so you better make sure you know you’re spending your limited time on the most important things.
On the other hand, the documentation you get out of the risk analysis process is an important tool for learning from the project after it’s done. It will let you look back at a decision you made and discuss whether it was the right one – and if it turned out not to have been, then you’ll know why you made that decision at the time, and what you need to change in order to make a better decision next time.