just to give additional charm to hands and feet and make them more formal and beautiful. These glitters are in different colors so can be used in matching with the dress. So you can choose the color according to your choice, event and dresses.
This is mostly used by bridals but now girls are also seems to like these glittering patterns and increasing trend in 2015. Saloons and parlors are specially using these glitters in mehndi. some pictures are also added in them which give a more formal look to it.