[quote=@Cyclone]
[@Kho]

As I recall, I questioned why the oceans were separated by isthmuses on that version of the map and received no answer. Beyond the addition of the ocean on the icecap those landbridges were the only change from the prior version, and since I don't remember the separation of the oceans I didn't officiate the map.
[/quote]

Vestec raised them during the collab where he made the world mountain and melted the caps so the sudden influx of water wouldn't flood the entirety of the world. 

They also serve as different ways of access to different parts of the world via land. At least they do in Vestec's mind.