Anyone whose a fan of the Fighting Fantasy Adventure Gamebooks might be surprised to know there were two different Steve Jacksons writing those books back in the day.
One was the co-founder of the series with Ian Livingstone and they wrote books both together and solo.
The other Steve Jackson was an American and despite having the same name as one of the co-founders the books made no efforts at all to differentiate between the two when it came to their separate works. Much to the confusion of numerous fans over the years.
I've considered using a pen name simply for reasons of privacy, not so much that I'm aware of there being existing authors sharing my real name, but there has been a very prominent local (to me) politician and a kickboxer who share my name. And even taken surname alone into account, there's been one very notable actor and a painter (whose art my dad can't stand) that people might think I'm trying to steal the thunder of or ride the coat tails of.
This might be why certain acting guilds insist on only having one member with a certain name, meaning that another identically named actor has to adopted a screen name.
After all, there's only so many names out there, there's bound to be other people by the same name you might bump into by accident. Just ask any one called John Smith.