c# - When do we need to put using directives inside a namespace scope?

I don't know why Asp.net MVC developers put the using directives inside System.Web.Mvc namespace as follows.

namespace System.Web.Mvc
{
    using System;
    using System.Collections.ObjectModel;


    [Serializable]
    public class ModelErrorCollection : Collection
    {

        public void Add(Exception exception)
        {
            Add(new ModelError(exception));
        }

        public void Add(string errorMessage)

        {
            Add(new ModelError(errorMessage));
        }
    }
}

When do we need to put using directives inside a namespace scope?

Answer

See Should 'using' statements be inside or outside the namespace?