To stalk someone
Some people create fake profiles in order to stalk the ones they are interested in without getting caught or recognized.
To avoid embarrassment
Some people create fake profiles to use them to initiate talks with people they don’t know. The main goal in such a case is avoiding getting embarrassed.
For testing purposes
Many people, especially programmers, create fake accounts to do certain tests. For example, a developer can create a specially made Facebook account to test something on Facebook’s Api.
To do the dirty work
Some people create fake accounts to use them to support their own opinion, get votes or even criticize others without getting their real self involved.
Spammers usually create fake accounts as they know that sooner or later Facebook will recognize and delete those accounts.
To be free
Some people create Fake profiles to be free to do whatever they want without involving their real profile.
To bully others
Some cyber bullies create fake accounts in order to bully others without getting caught.
Fear of displaying their real identity
Some people create fake accounts because they are too afraid to reveal their real identities to people online. (See What are the body language signs of fear?)
To have fun
Some people create fake accounts to have fun, do pranks or to joke with friends.
Becoming their imagined ideal self
Dr Chris Fullwood, a cyberpsychologist at the University of Wolverhampton, said that some people create fake identities to narrow the gap between their imagined ideal self and how they actually see themselves in real life.
To explore the environment
Some people create fake identities to explore new, certain environments without getting noticed or caught. A person could explore a dating app, for example, without revealing his real identity.
No strong sense of identity
If a person isn’t proud of who he is or doesn’t have a strong sense of identity then he might use a fake account most of the time instead of being who he is. (See How to tell if someone is stalking you on Facebook?)
Someone doesn't accept your Friend Request on Facebook? 12 Reasons Why They Didn't Accept Your Friend Request
If you unfollow someone on Instagram means you will no longer see their content in your feed.