1 They might want to check their progress
Nosey people might want to know more information about their friends or peers to compare their progress to them. A Nosey person might want to know how much his friend earns to compare his progress to that friend.
2 To feel superior
Some Nosey people do their best to know about the misfortunes of others. Those people usually do that to gain more information about others in order to feel superior. Once a Nosey person knows that a friend has problems with work he might feel better about himself.
3 They feel powerful when they get more knowledge
Some Nosey people collect information because knowing more about others makes them feel more powerful and in control. Those people usually try to dig for any kind of information they can find about others just to add them to their knowledge base.
4 They are looking for bad news
If a person is jealous of another person then the first might become Nosey hoping to find out any bad news about the second. A Nosey person might be asking for more because he just wants to get some bad news about a person he is jealous of. (See Why do people become jealous of others?)
5 They are unsatisfied with their lives
People who are unsatisfied with their lives or have nothing to do usually become Nosey. Those people try to find out what others are doing since they believe they are doing nothing useful.
6 They are interested in someone
A person will usually become Nosey when he gets interested in someone. If a person had a crush on someone then he might turn Nosey to know everything about that person.
7 To feel secure
When a person feels insecure about his course of life he might become Nosey in order to confirm his beliefs that everyone else is not doing well and so feel better. The Nosey person might just be asking for reassurance that everyone else has a messed up life.
8 To put others down
A nosey person can seek information to put someone down or to feel better about himself. While brave people struggle to develop themselves and be better the ones who aren't brave enough look for other ways, such as devaluing others, to feel better about themselves. (See What Makes a Person a Narcissist?)