Afaik, this causes no harm and ensures the singleton functions(such as Awake, Update etc.) are executed before other classes. This can be changed in inherited classes if needed, but in 99% of the cases, I would say that you'd want the singleton to execute first. -50 seemed like a reasonable choice due to certain built in classes utilizing up to -100(e.g. UnityEngine.InputSystem.PlayerInput) and this way we still get plenty of free room around the singleton.