The adoption of a pseudonym is a common occurrence in original fiction. Anne Rice, Christopher Pike, Dr. Seuss, James Herriot, Lewis Carroll, Mark Twain, and O. Henry are names that are more likely to be recognized by the reading public than the birth names associated with these authors. And the reason behind adopting a pseudonym vary, from needing to separate out the genres written by a single author to aligning an assumed gender-credibility to an author in a gender-bias time.
The use of alternate identities is also prominent in online engagement and very much a part of the fandom community. Pseudonyms, or fan names, are often adopted by participants in order to interact with one another through commenting, presenting fanfiction, or creating transformative works of common audio and video source material. And within these interactions you are more likely to see an exchange between MoJoLove and UpTheHill than Molly and Jill. With the expansion of social media and online communication, it's not uncommon to see handles, but these types of fandom identities pre-date this.
Fandom identities take on a life of their own, which may be vastly different from the person's offline life, and in such they often maintain an element of separation of the two identities. There are a number of reasons why a fan might take on a fan name / pseudonym or even multiple identities. In some instances, like original fiction authors, multiple online identities help to separate their different works in different fandoms. In other cases, the creation of an online persona creates a freedom in online communication practices that they cannot achieve in person. Other times, though, the use of fan names is done in order to protect the fan from real life repercussions, such as if a kindergarten teacher was found to be writing sexually explicit fanfiction. Maintaining that separation is essential in fandom culture, even if the real life name is well-known, and an outing of a fan is seen as grounds for ostracizing. While fan names may mask the real identity of a person, they are there for a reason.