Why burn books?

I've just started reading Ray Bradbury's book - Fahrenheit 451. From what I've read right now, it is about a government that wants to destroy books as books are considered illegal.

I think back to the Nazi regime when they used to burn books as well. Confiscate books from the Jews, make it into a bonfire and burn them. I wonder what it is about books that worries governments so much. Recently in India several persons were arrested and accused to be Maoist sympathizers because of the books they had at home.

Is it because books show people a reality that they never knew existed? Is it because through fiction it puts ideas in the minds of readers that they could never have come up with themselves? Is it because books contain our memories and along with it our very essence of being?

I wonder, with the coming of social media, do governments need to be scared of books anymore? Can the 140 characters of a tweet be more powerful than a book? Have we already burnt our very last book? What does that mean for the world we live in?