Answer: It makes paper money more difficult to counterfeit.
Step-by-step explanation:
Paper money can be a very valuable commodity to possess so criminals often try to counterfeit it so that they can have more of it without earning it. To counter this, government authorities that print cash put in a series of measures to enable counterfeit note detection.
One such way is the use of Optically Variable Ink (OVI). OVI shows different colors when the note is looked at from various angles. It is not very easy to procure so it is difficult for counterfeiters to get their hands on it and use it on fake currency.