What is benefit from using import and export attribute in mef using .NET 4.0 ?

i read on some sites bout mef framework and not understand any thing

can you tell me why using export and import attributes and what is befit ?

please show me any simple example ?

using System.ComponentModel.Composition;
using MefShared.Interfaces;

namespace MefAuthentication
{
    [Export(typeof(IAuthentication))]
    [ExportMetadata("AuthenticationType", "3")]
    public class ThirdAuthentication : IAuthentication
    {
        public string Authenticate(string username, string password)
        {
            return "Authentication Implementation 3 is invoked";
        }
    }
}
Here is export why use export here 

I already use inheritance 

so why use Export