I have one very good friend who writes macho SAS type thrillers, and has a stand-in for every signing, every photograph etc... because HE is a SHE... in this case it's about protecting sales - how many of those macho thrillers do you see written by women? Not so many...
As to why care - well, say I write a novel and it debuts, sells only 8,000 copies in the US - that's tiny, barely enough to warrent printing to be honest, so then the second hits, and I only sell 6,000 diminishing returns and all that... but the publishers have faith in my writing, and know that the right story will fly... unfortunately book stores won't buy a third book in huge numbers, they'll expect between them to only sell 5,000, accounting for the drop off again... so instead of calling the new book a Steven Savile novel, the publisher brands it and Alex Carter novel, and I become a brand new quantity all over again...
Means those book buyers don't tarnish the new book with my bad track record... if it was a major shift in direction, say I wrote a romance novel, that's another reason, to demark one kind of story from another so you don't get a lot of disappointed fantasy readers scratching their heads over my lovey dovey book