为什么要使用 IConfigurationSource 来包装 IConfigurationProvider？

Question

下面是来自asp.net core的一些代码：

public static class EnvironmentVariablesExtensions {
   ...
   public static IConfigurationBuilder AddEnvironmentVariables( this IConfigurationBuilder configurationBuilder, string prefix) {
      configurationBuilder.Add(new EnvironmentVariablesConfigurationSource { Prefix = prefix });
      return configurationBuilder;
   }
}

public class EnvironmentVariablesConfigurationSource : IConfigurationSource {
   public string Prefix { get; set; }

   public IConfigurationProvider Build(IConfigurationBuilder builder) {
      return new EnvironmentVariablesConfigurationProvider(Prefix);
   }
}

public class EnvironmentVariablesConfigurationProvider : ConfigurationProvider {
   ...
}

public abstract class ConfigurationProvider : IConfigurationProvider {
   ...
}

public interface IConfigurationSource {
   IConfigurationProvider Build(IConfigurationBuilder builder);
}

我的问题是，为什么 .net 团队使用 IConfigurationSource 来包装 IConfigurationProvider？去掉IConfigurationSource，替换任何需要IConfigurationSource到IConfigurationProvider的方法签名不是更简洁吗？ IConfigurationSource 的目的是什么？