According to some political scientists, interest groups are necessary because they give the American people a way to have their voices heard on a consistent basis.
Without interest groups, the only way government officials "hear" our voices is when we vote. This does not happen often and it does not send a clear message. If I vote for Mitt Romney this year, am I voting because I like his stand on abortion or his stand on taxes or, perhaps, am I voting simply because I hate Barack Obama? A vote for Romney does not tell us the motive of the voter.
Interest groups allow us to communicate constantly and clearly with the government. If I belong to the Sierra Club, I know that it will be constantly pushing for the environmental policies that I prefer. In this way, interest groups are a superior way of allowing us to tell our leaders what we want them to do.