Writers use pen-names for a range of different reasons, and I don't think I need to go over them here, but I can point you towards various lists of explanations, in case you're interested:
Writers' DigestFor me, there were a number of motivations. The main one is that I've published academic books using my real name. If a search for my name turned up the academic books as well as my novels, there's a chance that some readers will assume that the fiction is going to be more highbrow than it is, and be disappointed by the reality. Other readers, and probably more of them, might decide not to give my fiction a go if they think it's going to be more scholarly than it is. It's also just possible, should anyone like my fiction enough to search for it, they could end up with an over-priced dusty academic tome by mistake. It just seemed easier to maintain a separation between the two, though without any intention of keeping my real identity secret.

It never occurred to me that it mattered as an academic, but there are a few other authors already using my real name to write about a range of different things, including service dogs, a character called Whistling Jack, nuclear and advanced weapons technology, Montana Jewish communities and Jesus's conversations with women. None of those is me and it's probably best for all of us if we keep our publications distinct.
Another reason is that my own name is a pretty good indication of my age because it was one of the most popular names in the year I was born. That's fine in real life, because anyone I meet can see for themselves how very wrinkled I am, but it didn't seem sensible to flag it up for potential readers who are likely, on average to be younger than me. If readers get a couple of pages in and think my books are too fuddy-duddy for them, that's fine -- at least they've given them a chance.
My original intention was to use a pen-name to pay my respects to my grandmothers. When we were choosing names for our children, we decided to use family names, and we were lucky that there were some pretty timeless ones knocking about. It would have been much harder if we'd had daughters, because female names -- like my own -- are much more subject to fashion, and the names of grandmothers and great-grandmothers tend to sound like old lady names until at least a century later when they start sounding pretty and quaint again.
So the plan was to call myself Evelyn Gentles -- Evelyn being my maternal grandmother and Gentles the maiden name of my paternal grandmother. The first two people I told said 'What, like genitals?', which suggested that wasn't a great plan. I played around with a few alternatives based on family names, most of which sounded like what they were, which was two first names put together. Eventually, I settled for Frances, from my maternal grandfather's middle name, though he spelt it the male way, and Evelyn as the last name. I'd like to pretend to be honouring John Evelyn, but the honest truth is that I've never read anything he wrote. 

Naturally I googled to see if there were already hundreds of people with the same name, and it seemed as if there wasn't anyone who was likely to cause any confusion, which has to help if I want to come near the top in the results of online searches. I was pleased to see that the Countess of Warwick, the inspiration for the song Daisy, Daisy was born Frances Evelyn Maynard, and I give you that ear-worm as a gift.
An unexpected bonus of writing under a pen-name is that I've found it far easier to promote Frances's book than it would be if it had my birth name on the cover, not least because I can write about Frances in the third person without feeling awkward. Blowing one's own trumpet is, as the Lord Herald can attest, an unpleasant business.