I would say fairly important for the LNIB, and the lack of would come with a fair price reduction.
The more beat up the knife gets, the less important the box becomes.
It is only important to me, because it is important to others. If it lacks the box, I will likely get less if I sell it, and try to price my buying point accordingly. I will buy with or without a box - and much of what I have never came with a box from the maker.