As an editor I see a fair number of writers using pen names. These are just a few thoughts on the subject. Pen names got started for a couple of reasons. Women often found that men would not read stories written by women, especially in certain genres. And some people with ethnic sounding names were forced by publishers to use pen names much as many actors were forced by movie studios to change their names to more American sounding names. These issues are not so prevalent nowadays. What does remain is fans of certain genres can't cope with their favorite author writing in another genre. Mystery fans would never forgive somebody for writng in science fiction. Ergo, if a writer wants to do both, one of those genres needs to be done with a pen name.
I also know of a writer who does horror and fears getting static from the Mormon Church because his stoires have fairly strong sexual content--hence a pen name.
But that's not the case with most of the writers I see using them. Most of them are the most amateur green writers out there They've decided on a pen name that is often stupid (yes, the name itself is stupid) ordered their writer business cards and writer stationery and set out to be a writer. Everything is all planned out--except the bothering to learn how to write part.