Let's say you need to choose some technology for your project, or you already have problems with some technologies, so you want to replace them with others. If you do not maintain a decision log, you have a high risk of making bad decisions that are biased, based on authority, and take too long.
Yeah good point.
In that process-heavy project I joined, I could quickly see the problems; about 2 or 3 days. That meant when I was submitting code or reviewing the backlog I knew what kind of challenges I could make, and what would just be a waste of time. In others, it could take way longer - months! - to learn how the team actually deals with challenges and design.