One Deck-One Deck means you can effortlessly deliver a whole DJ set using just one deck and one button. as soon as you have mixed in the next song press the v key (or whichever button you wish to map this function to) and the deck will be duplicated and faded into the mix. meaning you can reuse the same deck for the next mix. 

lets say you are a scratch DJ who is most proficient with a certain hand ( i.e. every scratch DJ) you'll be able to use your dominant hand for every scratch & mix

or you are playing live and one of the controllers / CDJ's/needles etc. becomes faulty, with this button you won't have to freak out! 

alternatively you might just own one turntable at home and want to start scratching and mixing on vinyl / you want to streamline your setup or your decks are being serviced but you want to practice for a set, 

OR my favourite reason. because the idea of doing a set somewhere with just one laptop and one Deck is kind of amazing.