http://www.w3.org/QA/Tools/Icons
It's completely useless in my opinion - it wastes bandwidth, it serves no real purpose. The only people who really use it are the ones who just found about w3 validator usually, and they stop using it after awhile. The ones who do leave it on their sites usually don't even bother fixing validation errors after a couple years and you click on the icon and you see that the site has X # of errors.
And what is the difference between Gold and Blue logo
One is blue, the other is gold. The reference doesn't state any difference so I would assume it's just color.
Why W3C provide these logos?
For novice developers to have bragging rights and put something on their page.