On a large scale, let's say government, the idea is a brilliant one. Things such as numbers stations, and other media, have been broadcasting secret messages that are available to the general public for decades through things like shortwave broadcasts. Thanks to the use of one-time pads, however, only one agent in the field, or group, can decode these messages. They are absolutely uncrackable. It's just an easy way to make sure the intended receiver has a high chance of receiving the message when things like cell phones may not always be readily available. That's some serious covert shit though, so you're absolutely right. They're doing it for the same reason kids do everything else, it's "cool."