There's another benefit to having a decision journal, even if you do it alone and not as a team or company-wide effort: it reduces the time you have to waste fighting misinformation.

Companies bigger than a typical seed-stage startup usually come with an added level of bureaucracy. Products, tools, documents and others are usually created or pushed forward by different teams as a cross-functional effort. If you are the leader of such a project, the creator of a tool, methodology or document used by others, you will be expected to keep everyone in the gallaxy up-to-date with the details. Failing to do this will lead to certain personalities giving you a hard time. Suddenly people may start questioning your decisions in calls, emails and threads. You'll start wondering where all this skepticism comes from and will be forced to repeat yourself over and over again wasting valuable time. "Yes we did test whether to use X or Y architectures", "yes, we did test and consider this tool but it lacks feature Z".

I'm likely not alone in considering the above a less than ideal situation to be in. Having a well organized and detailed decision journal allows you to stop the noise almost completely. Also, as others mentioned, it gives your thought process structure, keeps you honest, committed and makes evolving a deliverable overtime easier.