[kandu]

I guess that those websites should serve the interests of the voters, not of the current politicians.

Why should a voter use such a website? Because his/her humble click on a Support/Oppose button could aggregate with many others' clicks and make a difference when the number of people expressing an opinion is high enough to get the attention of the politicians and of the media. The random expression of an opinion on the street influences just the few listeners that happen to be arround.