Community Promotion Ads seem to be a useful tool for helping spread various links that might be useful for users of the site.
I have mostly encountered them at MSE. I have learned several interesting things from them and I consider them useful. I do not know much about usage of them on other sites in SE network.
Do we want them here at MO, too? (On MSE, the questions for posting community ads were created by a member of SE team. So I suppose that, if we want to use them here, we need to ask SE team to start such a thread on this meta, too.)