When you're DJing, performing or even recording, sometimes all you need is a button. You could use a keyboard controller to cue up samples, but that's awkward - especially when MIDI pad controllers do it so much better. And they also do plenty of things your keyboard may not, like color-coded backlighting or direct software integration, all packed into a much smaller and more manageable console. If you're looking to build the ultimate DJ or production setup, you should definitely consider including a pad controller.
From DAWs to synthesizers, the possibilities for a pad controller are pretty broad. Some, like the Ableton Push Software Controller Instrument, are even designed to work with a specific piece of hardware or software. Push is a great companion for Ableton Live 9 - in fact, an intro version of that application is bundled in with the controller, so you can get started even if you don't already own a copy. There's no faster or easier way to get direct control over Ableton software.
You'll also find a fair share of MIDI pad controllers here with additional dials and faders, such as the Novation Launch Control XL or Akai Professional MPD32. These are great when you need a full set of controls at your fingertips, adding an extra dimension on top of what you can do with pads alone. Looking for something really unique? Check out the Korg Mini KAOSS PAD 2: it's a full-featured FX processor with a control pad, and they've somehow managed to make it small enough to fit in the palm of your hand.
No matter how you like to play or create your music, MIDI pad controllers are worth looking into. They can really power up your digital workstation, DJ booth or even an on-stage setup. With tons of opportunity for triggering effects, backing samples and full-fledged virtual instruments, these controllers are real workhorses no matter how you slice it.