A shared voucher code is useful for Facebook posts, embedding in websites, mass mailing lists - any time you want to reach a certain number of people and don't care how many times a given user uses the voucher. It's great for a First 50 people who click through get a free copy of my e-book type of promotion.
Conversely, if you're offering a give-away to a very specific set of potential customers with whom you would like to engage, a unique voucher might be more appropriate. For instance, you may offer five free signed copies of your hardback book to a blogger that they can advertise on their blog/website. The blogger can decide who gets each voucher - but when that person claims their voucher, the code cannot be used again. In other words, they can't pass it on to their friend to get a copy as well.