Spoilers & Speculation for Sylvanas | Part 1: Sylvanas uses the empty husk the dagger to trap N'zoth. N'zoth no longer in Azeroth is then sealed in the dagger & Sylvanas can use the power for herself with of course some inconveniences that Xalatath has done prior.
Spoilers & Speculation for Sylvanas | Part 3 / Alternative: I could also see Sylvanas potentially use the dagger as a method for escape so that she could live on in the dagger and one day escape and find a new body or vessel - this could backfire in a lot of ways too.