What's the difference between Request.Security.Tokens and Request.Security.Elements in WSE 3?

Question

So I'm trying to get a WSE 2.0 SP3 client to work with a WSE 3.0 server. I'm running into an issue with this line of code on the client:

transaction.RequestSoapContext.Security.Elements.Add(new MessageSignature(token));

I'm wondering how important is this line if the client is already using the following line of code:

transaction.RequestSoapContext.Security.Tokens.Add(token);

Does anyone know what benefits the first line of code provides over the second?