Lots of reasons! But for me, using pseudonyms helps my readers find my books in different genres. I write a lot of teen fiction, and I would never want a young adult to accidentally pick up a more mature book than expected. For that reason, I use the following pseudonyms:
M. J. Padgett
M. J. Padgett is my real name, so I've used it in most of my work. Since publishing my first book, I have reserved this name for young adult releases and anything rated PG-13 or lower. This is the name most of my readers look for when searching for my books.
E. J. Fleming
E. J. Fleming is a family name that dates way back to 17th century Scotland. Since The Assassin's Memoir series centers around assassins, spies, and villains of all kinds, I thought it only fitting to borrow my ancestor's name, considering he was a bit of a scoundrel. Anything R-rated in the thriller, mystery, paranormal genre will be under this name.