What is the security advantage of STS in web services?

I've 开发者_如何转开发started reading up on security (particularly authentication) with web services and I see a lot of references to security token services.

From what I see, they take a username-password (or something) and, on validation, return a digital token. How is using this token any more secure then just relying on the username-password in the first place?

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It's not necessarily "more secure".

It's just more convenient. Imagine you have more than one web service in your organisation. The client can use the optained token to prove his identity to all of those services.

I recommend reading this article for further information.