It might introduce new mechanics. For example, if the new story involves a new type of magic or power, there could be gameplay elements related to that, like new skills to learn.
They add depth. For example, when you play as Amber and know her story, you might play with more care and try to make her feel more important. It makes you more invested in the character.
The length of the Nilou story quest is not fixed. If you rush through it, it might take around 2-3 hours. But if you take your time, enjoy the cutscenes, and do all the side activities, it could extend to 4-5 hours or more.
As of now, the Genshin manga is not finished. The story is still evolving and there's more to come for fans to enjoy. The creators are taking their time to develop a rich and engaging narrative.
There's no definite answer yet. It might continue if there's enough demand and the creative team has more story to tell. But it could also stop if they feel they've completed the story arc.