[sonnyblarney]

"What does hacking into Yandex give us"

Russia does not need this or any other reason, they are already doing this to the extent they can. The public nature of this will provide some political cover, but that's it.

As to 'what it gets' it entirely depends on the kind of information that was obtained, the inherent risks and cost, the targets etc..

Maybe there were specific targets, a specific needs, maybe they were casting a net - who knows.