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There are a lot of posts from experienced webmasters on this forum which caution against the practice of link buying. You probably know that. So if your link buying actually helped, and didn't get you banned, sandboxed or whatever, then that's very interesting.
We pay for people to identify and submit the links to relevant websites. To the best of my knowledge it takes a while fo links to activate properly, which is why "paid" advertising links are an expensive option and therefore probably don't justify the money.
I think Google designed the delay simply to dissuade this practice.