^Well, I doubt there's any one person who watches 20 shows about the same thing, but there are at least tens of millions of people in a given major network's audience, some of whom might like some shows about a given thing but not like other shows about it, due to the cast and the characters and the writing/directing style and other considerations. For instance, I like CSI
but don't watch CSI: Miami
because I've never liked David Caruso. And I like Body of Proof
but got bored with Unforgettable
, and I like the cast of Castle
but I don't watch Bones
because I don't find Emily Deschanel appealing (and can take or leave David Boreanaz). So having multiple shows in the same genre strikes me as being about trying to achieve a balance between having an established network brand and having a broad enough appeal for a wide audience.
After all, is it really any different in other media? How many hundreds of murder mysteries or romance novels can you find right next to each other on any bookstore's shelves? They're all the same basic kind of story, and no one person has time to read all of them, but they're all there because fans of a given genre can still have different tastes and prefer some authors or some characters to others.