1) Because sometimes you are right about something and it's nice to show that you were thinking about it ahead of time. An example would be my prediction in 2023 (which is fairly late in all fairness,) that one of the most important improvements in AI in the near future would be the ability to interact with any GUI as if it was an API.

2) Because often you are wrong about something and having it written down and in public will keep you honest about hindsight bias. An example would be predictions I made about lasting impacts of the pandemic on how we approach healthcare and minimizing respiratory infections in general.