The profone answer probably is… unfinished content.
If we want to give the portals some meaning… The frost giants obviously are intelligent and probably able to use magic, as the frost forge shows us. They are said to be distant relatives, maybe degenerates of the giant kings. The giant-kings are summoners and are able to use teleportation magic through their obelisks.
It would be plausible for the frost giants to know similar magic. Personally, I don’t believe they used magic to travel to the Exiled Lands. If they are relatives of the giant kings, they’d likely live close to them, not travel to them for afar.
Neither do I see definite proof for the “snowglobe theory”. So, it’s most likely the forst giants used or used he portals to travel to other places and/or summon beings.
Do we see any evidence of them summoning anything or travelling to other places? No. There are no creatures except the frost giants and a lone white dragon in their lair. All the equipment they have - weapons and armor - can be created by the frost forge. So, it seems they live quite isolated, except maybe raiding the Cimmerians and Heirs of the North or even the Votaries of Skelos.
So… why? Maybe the fall of the giant kings discouraged them from using their magic? The giant kings’ fall was caused or at least accelerated by bad advice from a treacherous summoned demon. Maybe the frost giants understood what happened and now refrain from using their magic? Maybe, over time, they even forgot how to.
So, in short, I’d assume that the portals are unused, half-forgotten relics of an age gone by, when maybe, the frost giants were as powerful as the giant kings, and explored the world and maybe even places beyond this world.
P. S.: The portals are remarkably similar to the portals that leads to the Well of Skelos. There may be a connection: Maybe the Serpentfolk taught that magic to the frost giants that lived near them, or maybe it was the other way around.
Portals created by the Lemurians look differently, as we can see in the Dagon dungeon. They don’t have black, swirly spirals, but show the place they send you to.