This black form of glutinous rice is used in sweet dishes, particularly black rice pudding. It has a slightly more nutty flavour and stronger texture than regular glutinous rice, but the main aspect of it is the black colour.
To prepare this rice, you need to soak it for at least 3 hours to soften the grains before cooking. When you soak the rice, it becomes more deep purple than black, it will also discolour the water and any food it is cooked with.
When you need a black contrast to a white sauce (as in Thai rice puddings) this form of glutinous rice is perfect. However you can achieve a very similar flavour with regular sticky rice.