Why do guys grow their pinky nail long?

This might sound a little weird to you, but yes, various men grow their pinky nail long as it makes it easier for them to pick their ears. This is usually done by people of lower social classes in countries like China. (See What does touching your ear mean in body language?)

As per few cultures, long pinkie nail is considered as sign of wealth and fortune. Also, people who do palmistry say that if pinkie nail is longer than distal knuckle on his ring finger, it is considered lucky and intelligent. Numerous people grow their pinky nail so that they get better palmistry readings.

There are many regions where people grow their pinkie nail to show that they are not doing manual labor. It can also be said that they try to show off that they belong to a higher class. 

This might sound unbelievable, but they actually grow the nail of their finger to wipe their nose or scratch certain areas of their body.

This is true that various people grow their pinky fingernail so that they can take drugs easily, such as cocaine, etc. Many people think that this is a myth, but no people really do it so that it becomes easy for them to take drugs. (See Why are some rich people stingy?)

People grow this nail to use it as a tool or handle objects. This reason is a little strange, but people actually do this.

A lot of people grow the nail of the last finger as this finger is not used much. As it becomes difficult to use the hand if other nails are big so pinky is fingernail is the best to grow nails.

For making the pinky finger more useful, people often prefer to grow their nails.