I don't know much about DJing, but That gear in the pic is mostly just Turntables... But why people use other stuff along with a workststion? Some people use more than one keyboard, some use external Sound Modules, just to make music better... Some people use computers,(computers are mostly used as a Sequncer/Recorder) for sequencing cuz its easier for some people...
I don't even know why they use all that extra stuff... some people can listen to the music made with Keyboards and all the extra stuff, and they can make it almost exact just using the keyboard...
I know why Guitar is used along with keyboards, cuz you can't play a guitar on the keyboard and make it sound like real...you know...