Because all the cool kids are doing it.
So, I was perusing the Pit of Voles, and came across a Buffy S4 summary that had me asking "Why aren't there any Giles/Spike fics out there?" Huh, thinks I, and I promptly plug Spike and Giles into their little character engine.
After finding a summary that said Um, I really don't know how to explain this. Giles becomes young again, Spike gets captured, it's mpreg, it's slash. Oh just read it! lol. and having my brain explode, I cleaned the walls and put my head back together. Then there was this one: Set at the beginning of season 6; Spike invites Giles and Xander round for a drink. With Buffy and the witches out of town Spike is prepared to do anything to save Giles when he is kidnapped from his nose. After recovering from that very interesting mental image, I found one of Meltha's fics, here, about Spike and Giles and Dawn's first date.
Now, Meltha is a genius humor writer. I needed to glue my butt back on after laughing it off, and then I decided to see what else she'd written, because I hadn't looked at her fic in awhile, although she's on my favorites list. And, lo, what did mine eyes behold but a Buffy/Firefly crossover. Serenity has a stowaway! And it's not who you'd expect.
So, go, read. You'll be glad you did. It's called "Hitching a Ride."