1 For picking the ears
Some people grow the nails of their pinkie finger long to make it easier for them to pick their ears. This is common among lower social classes in some countries such as China.
2 To get better fortune
According to the practice of palmistry, a person is considered more fortunate if his pinkie finger is longer than the distal knuckle on his ring finger. Some people might grow the nail of their pinkie finger to get better palmistry readings.
3 The person doesn't do manual labour
In some areas of the world growing the pinkie fingernail is done to show that the person isn't doing manual labor. It's thus a method of showing off, for a person tries to convey the message that he is of a relatively higher class.
4 To clean their nose
According to a Quora thread, some people grow their pinkie nail in order to be able to reach certain areas of their body easily such as their noses.
5 To handle drugs
Some people might grow their pinkie finger nail to be able to handle drugs, such as cocaine, easily. Some people, however, consider this reason a myth.
6 To use it as a tool
Some people grow the pinkie fingernail to use it as a tool to do various things and to handle various objects.
7 The pinkie finger is not used often
Because the pinkie finger is the least used finger, it's considered the best candidate for growing nails as it would be hard to use the hand if another finger had long nails.
8 To enhance the use of the pinkie finger
Because the pinkie finger is not used often, some people might grow its nail to make it more useful.