Just an idea is a public idea web log; an informal registry for ideas. Everyone is welcome to add their own ideas to the site. Doing so, helps establish the born date of your idea and allows others to give you feedback.
If the person that originally conceives an idea just thinks about it and never registers it or tells anyone about what he or she is thinking, well then, at a later date someone else could get the same idea and make it public. Then, in a case like that, the person that originally conceived the idea probably wouldn’t get any credit for the idea.
Not only that, but once people are exposed to your idea, you could easily receive contributions or commercial offers to develop your idea. Besides, it’s just an idea; what do you have to lose?