This is like killing a person with polonium, which is made in just a handful of facilities around the world. Russia wants to cause worry. They want "respect", which mainly means fear.
Unlike the poisoning of Alexander Litvinenko with polonium-210 (to show agents don't defect and live), unlike military exercises in plain view of your adversary (to show your military prowess), and unlike displaying new shiny hardware at a parade (to make your adversary wonder about their capabilities); covertly tapping (or cutting) cables is done covertly, and as someone else mentioned has been going on by "both sides" for decades. If it was done overtly, the country who was being spied on would be tipped off and stop using that means of communication. I think willvarfar is closer to correct, they are probably mapping and looking for things, for future use, not exploiting things right now. I still think one of the things they're looking for, if they're actually over the cables, are other taps that have been placed by others before, or maybe looking for static ASW equipment laid there to detect subs.