From MoDo:
Pseudonyms have a noble history. Revolutionaries in France, founding fathers and Soviet dissidents used them. The great poet Fernando Pessoa used heteronyms to write in different styles and even to review the work composed under his other names.
As Hugo Black wrote in 1960, “It is plain that anonymity has sometimes been assumed for the most constructive purposes.”
But on the Internet, it’s often less about being constructive and more about being cowardly.