Hi! The devlogs are mainly used for updates. It emails everyone that has purchased the game. So if you update something in the file or have news about something to do with the game or just want to share some backstory, you're welcome to fill out a devlog, but it's never a requirement.
To do so you go into the edit game menu and look at the tabs at the top. The second one is devlog. You can fill one out there.
~Angel