What it does is prevent them from settling most of the whole peninsula, because they can't settle within two tiles. Asymptote (with 10 culture but that's a given from monument) gives us sight of their side of the water, so we see any ships, but on land it's all the way around on the other side. The "whales" city on the Cape is not going to be settled by either of us soon anyway I'm guessing.
Still, we can adjust to what we see, most important if there are mountain ranges or hills around, that makes a big difference. If the we see with warriors the center of the land east of Resonance is really lush/full of resources we could prioritize that; if it's a big old mountain range then all we have are the coastal cities anyway, and we're not settling the northern jungle yet obviously.
Again, I personally feel it's not all that necessary right away, but that's because I don't think the pressure to settle really fast anywhere on that petal against the Vikings is there. It's a city we want eventually though, and it is more effective at blocking/being a border than Resonance is, with the information we have right now.