Cheating is lame. Boosting is the dumbest cheat ever. Boost your stats for what? So you can get some poon by telling a chick your K/D ration? HAHA! There is no point to it in my opinion, if they want a fake stat for whatever reason let em have it. It doesn't change wether they are a good player or not. Only a complete retard would risk a ban by boosting.
To answer the question... basically it's just that: They want to look like they're good when people look at their stats.
My Opinion (because I KNOW you want to hear it ) A lot of people care about K/D, and it's sad. I mean hey, if you're good, it'll show in the after-game score board, right? If you have a 4.2 K/D, but you go .56 five games in a row... that kinda says something.
I've learned not to trust K/Ds, rank, or level as a way to see how "good" someone is. So many people either boost or used to suck but got better that I just don't care. And people who get a second account just to show they're "good" now? On Xbox, that's sixty extra bucks per year! And for what?
I personally wish it didn't show K/D at all, simply because I don't feel like it's an accurate guage for how good someone actually is, as the longer you play, the less it changes due to the law of averages, plus it encourages what (almost) everyone hates - boosting.