I see that when connecting services to twitter/facebook, sometimes apps are storing the user access token in the server. What is the most common purpose of doing this? I've read several twitter/fb documents and just don't seem to get it. Twitter also has an xAuth which can basically provide you with the user 'access_token', so what is the purpose of doing this?
You don't want 3RD parties pretending to be users. You want signed transactions so that authenticity is validated.
What if some application just started posting shit on your wall? This has a pretty obvious purpose.
Your app has to provide the token when making API requests that have been approved by the user. That's what OAuth is made for.