It's common enough -- recently I was looking for Seanan McGuire's newer book only to find it was published as "Mira Grant".
Why would an author do this?
Two reasons I know of:
There may be legal reasons, too - but IANAL.
Women writers used to do it because only men authors were taken seriously. Sometimes people don't want the fame from their writing they just want to do it for the art. The pen name allows them peace from the hype of their book. Sometimes people are afraid of critics and feel better if its not actually their name being bashed it makes it feel less personal.
Sometimes pen names are used to fit with an imaginary "true story". A great example is The Princess Bride, in which the real author (William Goldman) pretends it's a "true story" written by someone of the era (S. Morgenstern).
Still, that falls under Marketing I guess.
Another reason is that an author may want to be shelved with other authors of their genre, especially highly successful authors. For instance, if you were a fantasy author it might seem attractive to have your novel sitting next to Tolkien's great works. Thus, anyone looking for Tolkien may stumble across yours.
On Stephen King's website we learn that he used a pseudonym to be able to publish more than 1 book a year during a certain period of his carrer:
"I did that because back in the early days of my career there was a feeling in the publishing business that one book a year was all the public would accept [...]."
Zane hit the main ones: desire for personal privacy the other primary reason I know is marketing - same as actors, some authors will adopt snazzier-sounding names to sound good on the bookshelf.
Beyond that, you've got a lot of exceptional cases - Joe Hill is a pen name to avoid the otherwise-painfully-blatant connection to his father; Alice Bradley Sheldon probably falls under "privacy" but in a very extreme manner, etc. etc.
One unusual reason I haven't seen touched on: Some people do it to access a side of them that they want to express, creating a kind of virtual "person" with different attributes. Some people call it their "muse" and other pet names.
It's a way of allowing themselves to overcome some mental barrier by pretending to be someone else, with special "powers", like a kind of super writer with confidence they might not normally possess.
Personally, I don't go in for it ;)...
To answer your specific question about Seanan/Mira, see http://seananmcguire.com/writefaq.php#mira. In her case, she's using different names for urban fantasy vs science fiction.
There's also the classic midlist death spiral--author doesn't sell enough, publisher drops them, author changes name so they can sell new books.
Besides "personal privacy" there is "professional privacy", an author that holds a sensitive full time job, in public, may not want to publish under their real name because the content of their fiction may have some impact on their professional life.
Their fiction may, for example, contain explicit blow-by-blow sex scenes, or portray homosexuality or drug use in a positive sense, or implicitly endorse crime (like Mr. Robot does hacking) or murder as solutions to problems.
Especially in the current social climate, we cannot trust people to distinguish fiction from the author's real views, or indeed the fiction may contain the author's real views, which they do not espouse in public because they would embarrass their institution, company, or office, and in many high level positions that can be a termination offense. For an elected official, it can be fodder for their lying opposition.
Examples are professors, anybody elected to any public office (even City Council, Sheriff or City Manager), medical doctors, lawyers, mental health professionals, CEOs of various companies, even people elected to private office; like The Quilting Club presidency or a local Credit Union Board of Directors.
This may not be a strategy to take to the national level, but some of us are self-aware enough to know we aren't going to be running for Congress or President or even Mayor, so nobody is going to hire private investigators to do a deep dig on our finances.
Of the Brontë sisters' motivation to use pseudonyms Wikipedia has to say:
In 1846, the sisters' poems were published in one volume as Poems by Currer, Ellis, and Acton Bell. The Brontë sisters had adopted pseudonyms for publication: Charlotte was Currer Bell, Emily was Ellis Bell and Anne was Acton Bell. Charlotte wrote in the "Biographical Notice of Ellis and Acton Bell" that their "ambiguous choice" was "dictated by a sort of conscientious scruple at assuming Christian names positively masculine, while we did not like to declare ourselves women, because... we had a vague impression that authoresses are liable to be looked on with prejudice[.]"
Reasons that I've heard of: (I'm overlapping other posters here somewhat but I'm trying to be complete.)
The author doesn't want family, friends, or business associates to know that he is writing this sort of book. The most obvious example would be if he is writing pornography. This includes other controversial subjects, like the author has a job where most of his co-workers are conservative and he wants to write liberal political books, or vice versa. Or the author wants to write about controversial religious beliefs, etc. Or it could simply be that someone trying to establish a reputation in a "serious" profession fears that it would look bad to write "frivolous" books, like a college professor might not want colleagues to know he writes escapist adventure stories.
Protect privacy. Some authors love becoming celebrities and getting attention from fans and the media. But others hate the idea, they want to shield their privacy. So they write under a pseudonym so people have a hard time tracking them down.
The author believes that a pseudonym "sounds better" or sounds more appropriate to the genre. Like someone writing about French cooking whose real name is Lin Chang may decide that that name will not bring French cooking to mind, and so write under the name, say, Francois Durand. Or someone with a foreign name that is hard to pronounce may use a pseudonym that is simple and easy, like Fred Smith. Or conversely someone with a very common-sounding name, like Fred Smith, might use a pseudonym that sounds more distinctive.
Authors who write in multiple genres sometimes use different names for different genres to avoid confusing or offending fans.
Writers for small magazines or newspapers sometimes use pseudonyms to make it look like the publication is bigger and more diverse than it really is. If someone is trying to start a new magazine and the first few issues consist solely of articles by himself, he might use pseudonyms to make it look like there are a dozen different writers.
To differentiate between genres, averting confusion, especially in marketing.
Hardly a pseudonym, but there's Iain Banks - fiction writer and Iain M Banks - science fiction writer. Collectively, these days, he's known as Iain [M] Banks.
Publishing companies usually place restrictions on how many books their authors can publish a year. Writers are restricted to just a few books per year by their publishers. One obvious way for writers to get around that is by submitting their work to other publishing companies using pseudonyms. And many of them do that - especially the more prolific ones.
For example, to distantiate themselves from what they are paid for to write for good money from what they want to be proud of.