Low and stable inflation gives people  and firms predictability. Knowing that inflation will remain at around two percent makes it easier to make economic decisions, plan large purchases and make investments. Low and stable inflation is a precondition for a well-functioning economy and for keeping as many people as possible employed.
There is no absolute answer for determining the optimal level of inflation. Over time, however, inflation should be high enough to prevent prices from falling, and low enough to prevent a self-reinforcing wage-price spiral.
Norway and many other countries have therefore chosen to target annual inflation of two percent over time. By choosing a point target for inflation, Norges Bank and everyone else have a nominal anchor to aim for and refer to when planning for the future.