the whole security model of sudo makes no sense

I think that is a bit strong. Sure, you aren't gaining much protection if you just allow sudo -su root but there are a lot of valid use cases.

1. Logging.
2. A bit of an "explicit" check to keep you from doing something stupid without thinking.
3. You can configure sudo to only allow specific commands from different users. (Maybe a trusted friend should have permission to reboot your Minecraft server but nothing else)