Why? Because they're fun to write, and to read. Because I get to come up with a background theme and some characters and story lines and incorporate them into more than one book. I start with one as a basis, then do more. Hopefully, a lot more.
There are many kinds of series, of course. I write three different sorts: cozy mysteries and two mini-series in romance lines: paranormal and romantic suspense. Are they the same? Sometimes. But in other ways they're very dissimilar.
These days, the underlying theme in nearly everything I write includes dogs--although not in all my Harlequin books... yet. Otherwise, each series is quite distinctive.
For example, my upcoming UNLUCKY CHARMS is the third in my Superstition Mysteries, one of two cozy mystery series I currently write for Midnight Ink. Its background is, unsurprisingly, superstitions. My protagonist, Rory Chasen, goes to the fictional town of Destiny, California, which is all about superstitions to learn more about whether the reason her fiancé died after walking under a ladder really was a result of a superstition coming true. She brings her dog Pluckie with her, and learns that black and white dogs are lucky after Pluckie saves the life of the local pet boutique owner. That was memorialized in the first book, LOST UNDER A LADDER. Rory stays in Destiny to manage the pet boutique and remains a superstition agnostic, although superstitions do seem involved in solving the murder in book two, KNOCK ON WOOD... and also in book three, UNLUCKY CHARMS, which will be an October release.
So what's the same in this series? The theme: superstitions. The location: Destiny. The characters: Rory and Pluckie and others, some of whom are also introduced in the first book including Justin Halbertson, the local police chief and Rory's love interest.
And what else makes it a series? Someone gets murdered in each story and Rory has to solve it, no matter what Justin tells her. There's always a resolution to the mystery, though not always to the budding romance.
Since I write multiple series at the same time, I've developed ways of keeping track of pertinent details in computer files, such as characters and their looks and traits, locations, and in this case the superstitions I've used. I do the same kind of thing, as appropriate, for each of the other series as well.
I also write another cozy mystery series for Midnight Ink, the Barkery & Biscuits Mysteries, in which the protagonist, Carrie Kennersly, a veterinary technician, owns two bakeries--one for humans and one a barkery for healthy dog treats. She, too, solves murders. Again, there's an underlying theme: the bakeries. The location is the fictional town of Knobcone Heights, California. And again, the characters like Carrie, her brother Neal, and her romantic interest Dr. Reed Storme, a veterinarian, show up in each story.
Then there are romance series--well, actually, mini-series since mine are part of a larger group of stories published each month.
My most recent Harlequin Romantic Suspense book, COVERT ALLIANCE, an August release, is somewhat part of a series, too. The series idea began with the introduction of the fictional Identity Division of the U.S. Marshals Service in COVERT ATTRACTION. The ID Division creates new identities for people in trouble who can't participate in witness protection because their knowledge isn't enough for them to testify as a witness against the bad guy--no hard evidence and only hearsay, no eyewitness testimony. The ID Division also sends someone undercover to try to obtain the missing evidence. In both stories, the protected non-witnesses are women who have critical reasons to return to their home towns even though they're under orders not to--and they meet the undercover guys sent there to dig up the evidence. Yes, these are romances, and you can guess that they fall for one another.
The series stuff? A theme: non-witness protection. A romance: they're part of a Harlequin series. But because they're romances, that theme is the driving force for the series; each relationship has to evolve to a happily-ever-after by the end of the book.
I also write another long-existing series for Harlequin, the Alpha Force paranormal romance series for their Nocturne line. Alpha Force, a covert military unit of shapeshifters, is what connects the stories and makes this a series; each involves one or more shapeshifters. And yes, since some of the shifters are werewolves who, as members of the military unit, also have cover dogs, my personal underlying theme of dogs is met here.
I've also written a couple of other cozy mystery series: the Kendra Ballantyne, Pet-Sitter Mysteries, in which Kendra, a lawyer, lived in the Hollywood Hills with her tricolor Cavalier King Charles Spaniel Lexie. I live in the Hollywood Hills and, at the time, was a practicing attorney--and my older Cavalier, Lexie, is a tricolor. However, I don't pet-sit all kinds of animals, nor do I trip over dead bodies. But you can see some of the theme here. And my next series was the Pet Rescue Mysteries, a spinoff from the Kendra books, where Kendra's wealthy boyfriend Dante funds a wonderful pet shelter in L.A.'s San Fernando Valley, and its owner Lauren Vancouver, like Kendra, keeps having mysteries to solve.
So, you get it. As many different kinds of series as there are, they're all similar in some respects like recurring story themes and characters and more. Reading one story in a series will hopefully capture a reader's attention and encourage reading all of them, so readers, too, like series.
Plus, in each case, the writer needs to keep track of all of the appropriate details to try to ensure consistency.
And as a writer? Well, I think you can tell how much this writer enjoys writing series novels!