Necessary of a purchase means there is a situation to purchase even , not push you to purchase the goods or services.It means the situation induce you to purchase the goods or services even, not push you to a demand for the good.
so you can leave from purchasing of this goods or services through your decisions such as ,you go to town by bicycle or private taxi.
But the necessity is the must.Because there is a situation however you must have to purchasing the good or services.such as medical goods or services .
Saving of money, for your future depend on your decisions.