I personally think we developed language because of our deep inner need to complain. You've just written your first novel, short story, or pornographic screenplay . You think about trying out that nickname on your manuscript instead of using your real name. Or perhaps you just don't think your birth name, Johann Bartholemew Snivelhopper III, has what it takes to get your manuscript into the arms of the magic Printing Press Fairy. Yes, you do.
Some authors use pseudonyms for the wrong reasons.
One possible reason for authors not wanting people to read work written under their real name is because they're embarrassed about what they've written. Then, by all means, use a pseudonym and wear dark sunglasses in public. Other writers consider using pseudonyms when they're discussing a controversial topic and fear disapproval, confrontation, or having blunt objects thrown in their general direction. Writers who tend to bleed easily or are not willing to defend the opinions attached to their real name should simply avoid writing about the subject entirely. Writers who aren't confident enough to support the words they've written can always find work writing warranties for used car dealerships.