Pseudonym - Who are you, really?

There are lots of practical reasons for using a pen name. Alice Sheldon renamed herself James Tiptree Jr to get past a bias in SF against female writers. Paul Linebarger became Cordwainer Smith because his role in international affairs and psychological warfare might have been compromised if some people had read his novels. When Nora Roberts is not writing traditional romances, her name becomes JD Robb, and that spares her readers confusion.

All of this is valid and provides some fun for those in the know, but, often, a pseudonym is freeing. Obviously, for those who write edgy work, like erotica, the anonymity can be of value. (It can be essential for those who write both erotica and kids' books -- yes, I know these people.) But I'm looking beyond that. A pen name can allow an author to discover and explore a different part of him or herself.

• A character - Cary Grant (Archibald Leach) said, "Everyone wants to be Cary Grant. Even I want to be Cary Grant." All writers like to create characters. Why not write as one? Using a pseudonym provides the opportunity to build a complex character and to use that character as a vehicle for creating art.  
• A focus - Your name might not be appropriate to your chosen genre. Mario Puzo probably wouldn't have worked as the name of a romance writer. Could you imagine picking up a picture book written by John le Carré? Would Delilah Marvelle, a regency romance author, make sense as a writer of thrillers? The right pen name can brand an author for readers and help the writer (especially one who writes in different genres) get into role, switch perspectives, and find an appropriate voice.
• A might have been - We all have made essential decisions in our lives. Using a pen name is an invitation to try out a different life, one that has very different boundaries and strings attached.
• An alter ego - A pen name can become a springboard for exploring a dark, embarrassing, or extreme aspect of ourselves. (Or for dark people, the opposite, perhaps.) 

This last is the most intriguing because it provides permission that might otherwise be lacking. And, without the inhibitions and expectations tied to the personas and obligatory roles we have in the real world, something new -- and authentic -- might emerge.