Well, legal documents are required if one wants to obtain something huge and expensive like a house or a car, since here legal basis is needed not only to make sure the money will not be stolen, but that the person we deal with has the rights to provide us with a certain product. If we talk about paying salaries, however, smart contracts are a perfect way to ensure people don't work for free. Smart contracts can also be of use if one buys something minor like pills or toys. Once you get them, you are sure the money wasn't wasted on scam.
Smart contracts could be very useful in real life, but for now they are still to hard to be performed and thus adopted massively by people.