Atmospheric focusing is a pretty inefficient process, on top of the existing inefficiencies of massive nuclear weapons it's probably not worthwhile to use as a weapon. It's a lot easier to send conventional bombs or bullets to Finland if you want to break windows there than to use multi-megaton bombs thousands of miles away.
The main reason you build big bombs is (aside from the arms race / show boating aspect) to destroy important targets with inaccurate delivery systems. The other reason to build big bombs is to intentionally create fallout, which could be used in a "doomsday" weapon, possibly as a MAD deterrent. If you exploded a 1,000 MT nuke on a containership in international waters off the coast of Europe or North America and you took advantage of the prevailing winds you could do devastating damage to the population, agriculture, etc.
The main reason you build big bombs is (aside from the arms race / show boating aspect) to destroy important targets with inaccurate delivery systems. The other reason to build big bombs is to intentionally create fallout, which could be used in a "doomsday" weapon, possibly as a MAD deterrent. If you exploded a 1,000 MT nuke on a containership in international waters off the coast of Europe or North America and you took advantage of the prevailing winds you could do devastating damage to the population, agriculture, etc.