The problem is that I need to know HOW this class does something, especially when debugging.

The unnecessary branching makes me need to keep an eye on the stack, and a lot of extra information in my head.

The additional state (in this case @email and @user) needs to be tracked from a distance. Local variables are 100x better. If there's any mutation, I must watch out during debugging two or three levels deep. Often there are more variables than that. If I had the "role" and the "permission" as a parameter, I would need two extra instance variables. With a regular function this is just two arguments, a couple lines away from the usage site.

The original code I wrote into ChatGPT to generate the code above shows me the what and the how with much more clarity:

      def create_user(email)
        user = User.create(email)
        user.roles << Roles.find_by(name: 'admin')
        user.permissions << Permissions.find_by(name: 'invite')
        Templates.find_by(name: 'invite').call(user: user)
      end