Sometimes a writer takes on a subject that is highly controversial or threatens to expose secrets. Rather than remain anonymous or risk personal injury as the named author, you may want to use a pen name for protection. The pen name you choose may reflect some element of the controversy, or establish your credibility as an expert on the subject. In a different vein, you may choose a pseudonym based on a character you enjoy from literature or history. A well-chosen pseudonym can add an air of mystery or intrigue.
Pseudonyms can create 'identities' from which there is no escape.
In the same way that an actor may be linked forever with a popular character, an author may be linked forever with the character created by a pseudonym. Some authors may create elaborate backstories to give dimension to their alter ego pseudonyms. In essence, the pseudonym's fantasy personality may overwhelm the actual writer's persona. Many authors who use pseudonyms discover that they must keep using that pseudonym long past its usefulness. Efforts to publish works under their real names may be met with rejection or reluctance.