This is for future-proofing. Occasionally the plugin's been broken by API changes that we weren't prepared for (I, at least, am not very good about keeping on top of announcements), and this way we are explicitly declaring which version we want and gaining a 24-month grace period to adapt after a new version is released.
https://docs.github.com/en/rest/overview/api-versions#about-api-versioning
This is for future-proofing. Occasionally the plugin's been broken by API changes that we weren't prepared for (I, at least, am not very good about keeping on top of announcements), and this way we are explicitly declaring which version we want and gaining a 24-month grace period to adapt after a new version is released.