The Recorder database is where all the “history” and “statistics” data are stored. The problem is, HA is advertised to work well on a Raspberry Pi platform using an SD card to store the data. And it does work well. Until the continual database writes wear out or fill up the SD card. Personally, mine has been running for over five years now on an SD card. Other folks run HA on different hardware platforms, and even different database engines. For them, the Recorder database is a non-issue.
For some reason this isn’t well explained to new users. It’s very easy to shoot yourself in the foot, just following the recommended installation instructions and taking the defaults.
What I think needs to be stressed is that users should carefully select their Recorder settings in configuration.yaml, and consider whether each entity they add should be included or excluded. There’s a good thread on this here.