1 A way to keep fans updated with their news
Twitter allows celebrities to instantly update their fans about their latest news.
2 The Following/Follower relationship
Celebrities like to be connected to their fans with no strings attached. Unlike Facebook where a person has to add someone as a friend to be connected to him Twitter allows people to update their fans without being friends with them.
3 To have a direct connection with their Fans
Some celebrities like to use Twitter to be directly connected with their fans and to be able to get direct feedback from them on a personal level.
4 To manage their popularity in a digital age
Most celebrities can't afford not to have a strong social media presence in a world dominated by technology.
5 Having followers satisfies the Ego
When a person get's more followers he feels good as his Ego gets satisfied. Because Celebrities get many followers they feel good when they use Twitter.
6 To strengthen their brand
Many celebrities use Twitter to build or to strengthen their brand. (See Why celebrities earn so much money?)
7 Facebook is too private
Facebook is a private social network. Because celebrities don't want to get everyone in their private lives and because Facebook has a maximum limit of friends they use Twitter to connect with their fans.
8 Quick way to update the masses
Twitter is a fast and quick way to update a very large number of people with something. It appeals to celebrities because it allows them instantly to share something with all of their fans.
9 They make money out of it
Some celebrities make money out of their Twitter account. It's believed that some celebrities get paid to make certain tweets. (See Why do celebrities use Instagram?)
10 To prevent others from speaking on behalf of them
When a celebrity doesn't create a social media account like Twitter an impostor can fake an account and claim that he is that celebrity.