That's the function they serve. It's not just a video game conceit; it's an actual portal that connects to other portals, but only when you (or at least one person in your party) knows where they are. Why does it work that way? I don't know, it's magic. This is a fantasy game.
But aren't the netherese portals just a "video game conceit", actually? Right now, only the PCs (including companions) even
notice these portals, let alone use them. Why don't the tieflings use them to travel? Why don't the druids use them to aid in bringing back forage? Why doesn't Minthara (a True Soul, if you want to claim that only those with tadpoles can use these portals) use them for hit-and-run tactics? They seem to exist just for player convenience, and thus are closer to an out-of-game-world mechanic than a true part of the world.
I'd be more fine with the portals if there was some in-game explanation for why only our party can see and use the portals. Even a single line from Gale "it seems to resonate with our tadpoles...hmm there's some hidden magic here that is allowing us to see these sigils!" would be nice.
For what it's worth, the game tries to stop that from happening elsewhere. Notice what happens if you follow Nettie into the back chamber and then you end up killing her while the stone doors are closed. The game won't let you fast travel via the waypoints because there's no way to get to a waypoint to initiate the teleportation. You have to figure out how to get out of the room first, by opening one of the doors.
That's good. I'd like to see more of this.
Why is it different ? It's exactly the same to me.
A convenient magical runes that allow you to go where you want and when you want even if there are no pathes (or if you haven't found one yet).
If they can teleport through the Netherese waypoint to the forge, why couldn't they to the surface or anywhere else ?
Don't get me wrong, I agree that we probably shouldn't be able to teleport so easily to the surface when we're in the underdark. But in my opinion it doesn't make more sense that we teleport to cross walls / lava / obstacles.
A key point about the portals (and any teleportation circle in D&D lore) is that you have to know the destination. So it somewhat makes sense that, once Lae'zel has discovered the waypoint rune, that she+the party can all teleport to a common location, then she can teleport the entire party to the new waypoint.
