Since tons of plastic and paper cups used at events end up on the ground rather than trash cans, these cups are designed to be tossed right on the ground, where the seaweed-extract they're made from helps plants grow. Consider that...the grass at a festival site being as healthy or healthier after the event that it was before!
They look somewhat fragile, but as long as you don't take too terribly long to finish your drink on a hot day, they'll probably work just fine. And without a doubt, they would account for less waste than paper or plastic cups.
Source: treehugger.com