What's not to understand? A keyboard and mouse simply offers greater and more precise control compared to a controller in most types of games. I'm sure there are some wannabe-intellectual pricks out there who will call me out as some kind of elitist for stating this, but I find it to be true (and frankly, the only elitists here are the people who think calling other people elitists somehow makes them better).
Obviously it's not going to be the best input for EVERY genre - fighting games and platformers tend to play better with a controller, and I wouldn't play a realistic flight simulator with anything but a popper joystick. But for anything where aiming or using menus is important - this would include FPSs, TPSs, RPGs, RTSs, MMOs, and a host of other gametypes - the mouse and keyboard is superior. And even then there are exceptions. Mirror's Edge plays just fine with a keyboard and mouse, but Halo always plays better with a controller.