> I wonder why do we need new notes...

An average lifetime of a banknote is 1-2 years for smaller amounts, 2-4 years for bigger ones. It is estimated that 15-25% of all banknotes are replaced every year. From about 30 billion individual banknotes in circulation that makes 4.5B to 7.5B each year.

So why not introduce new designs (and new protections?)