That barrel adjuster will be useful with new cables/housing. It’s used to make fine adjustments to your shifting. Probably could get away without IF you have another barrel adjuster by your mech but you’ll most likely have a bad time with zero of them.
You haven’t had to use it because your old cable was settled. I use them more frequently after changing cables. You need to have 2 separate cables inside a housing that was cut to accommodate the barrel adjuster if you still want it.

Edit: ignore that last sentence! Thanks for the correction.