Electricity is just that much cheaper in some places than others. Bauxite is shipped far across the ocean to places with cheap electricity for the energy-intensive process of refining it into aluminum. Sometimes it makes more sense to ship the electricity instead. Here in Canada, I pay like 20% of what the Japanese pay and 10% what the Danish pay for electricity. If we could figure out a way to export our hydroelectric wealth to regions like Europe and Asia, it could help climate change. High-temperature superconductors don't get enough research, IMO.

There is (if it has not been scuppered by brexit) Icelink, a project to connect Iceland to the UK. The rationale, as usual, is mismatched supply and demand, where the supply is green geothermal power.

https://askjaenergy.com/2018/04/17/icelink-in-operation-by-2...

The idea of connecting solar farms in the Sahara to Europe under the Mediterranean is also perennially in the air, with the same rationale as Icelink.

Some seem to be for redundancy, some bring power to municipalities (i.e. island cities) with limited power production capacity. There’s a list here https://en.m.wikipedia.org/wiki/Submarine_power_cable

Not all of these lines go across the ocean. If you're trying to move power between São Paolo and Beunos Aries, it might be cheaper to lay a sea cable than to deal with all the terrain, land rights and borders involved in an overland route.

Not all submarine cables cross oceans.