It's making content blocking extensions work the way they do in Safari. The benefit is security, as it allows for a massive reduction in the amount of extensions with unlimited powers to steal authentication cookies, MITM ostensibly secure connections, etc. The downside is a loss of flexibility in what ad blockers can do (though Safari ad blockers seem to get by well enough). It's unclear whether it's an improvement or reduction in power efficiency.

In typical internet mob fashion Apple doing this is considered doubleplusgood, Google doing it is them being evil and trying to kill the Internet.