Why govt trying to pass law to link aadhaar card to voter ID

On December 20 the bill has passed by the lok sabha to link Aadhaar card with the voters id many opposition party was against they stated that it can be affecting personal space of the citizens but ruling party clearly mentioned that the law which is passed is voluntary not mandatory but they have to clear this out this in detail if citizen has to follow this rule they had to give us a time.