Soap security negotiation when using wsHttpBinding

I was creating wcf service in .net 3.5 and used basicHttpBinding, with this configuration service running fine. If I change binding to wsHttpBinding it gives me error "soap security negotiation with for target failed".

Then I have specified <security mode="None" />,  and the service running fine now.

But I am unable to understand, why we need to use this.

Please clarify.