The demonetisation no doubts helps in eliminating counterfeit currency.
It also has limited scope to reduce black economy by attacking the person holding black money in physical currency. I say limited because the black economy is spread into gold, real estate.foreign exchange and other assets which are not enlisted by the govt.
A better step would have been to completely eliminate high value currencies and curb black money hoarding. Hiding a lot of cash is very tough in small denominations and even usage is tough.