There are many reasons for using a pen name. One is privacy. That's my main reason.
The primary reason, however, is that some writers are very prolific, and may write in two or three genres. Even if a prolific writer sticks to one genre, a single publisher may not be able to handle the output, but will not allow the writer to publish similar books with another publisher under the same name.
And if you do write in more than one genre, pen names allow readers to know what they're getting from the name alone.
While new writers, and even some pro writers, may take a year to write a book, or two or three years, it's a different story when you can write a book in four months, or one month, or, if you have to, in two weeks or less. I know one writer who has, twice, written 90,000 word novels in a week.
I've been in tight spots, and I've written 12,000 words a day for days on end. Can't do that for long, but some writers can do that, and more, endlessly.
When you turn into a well-established, bestselling pro, you may be able to drop the pen names, as Dean Koontz did, but early on, pseudonyms are often an absolute requirement for a very prolific writer, especially if he's taking on every job he can get, writing for everyone he can. That's why Koontz used eleven pseudonyms early on.
Pseudonyms are also useful for prolic short story writers. Or used to be, when there were more fiction magazines. One entire issue of a well-known SF magazine was filled with stories by the same writer, each with a different pseudonym. The funny thing was all the letters from the readers saying how much they like one writer, and hated another.
I don't spend seventy hours per week at the keyboard as Dean Koontz often does, but I do use a different pseudonym for each genre I write, and I've had two short stories in the same issue of a magazine, each with a different pseudonym.
Many wrietrs also use pseudonyms because their day job demands they do. This is more common than you'd think. It's pretty much mandatory in many fields.
Pseudonyms are also necessary when writing a book that will sell better with a name of the opposite gender on the cover. I once wrote a cetegory romance novel that had to have a pseudonym on the cover. I'm currently writing a western romance that also demands a pseudonym.