[giva]

I think they are mostly used when an AC trasmission is impossible: AFAIK only synchronized grids can be connected by AC.

https://upload.wikimedia.org/wikipedia/commons/6/6d/Electric...

[emilfihlman]

You are entirely correct. Power between different AC regions is mostly transferred using HVDC-links because it avoids synchronisation requirements between them.

Additionally the capacitive losses of an undersea HVAC cable are prohibitive, leaving only HVDC as an option.