There is a lot of controversy around interest groups, and just like any other subject, it has both good aspects and bad.
One positive of interest groups is that opposing groups can balance each other out. One major opinion is not always keeping and enforcing power over the whole group because another group can oppose that major opinion.
Another is that interest groups can promote grassroots movements. Grassroots efforts can bring the public together in order to create and promote legislation for the cause that they promote. Grassroots movements are also generally more accepted by the general public because while they do have an agenda, they are not spurred on by the government.
Interest groups can influence all branches of government legislation to pass; executive lobbying and support gained; taking cases to court.
By pooling the funds from thousands of people, those of little economic standing can still have a say in issues that they are passionate about. We all know that money is what ultimately decides most political agendas, so middle class Americans like us can have more sway when we join forces than if we go it alone and try to have a say with less economic backing.
Interest groups provide more ways for citizens to participate in government. Technically our main government participation is in voting, but with interest groups the average citizen can introduce policies to congress and see them enforced through lobbying.
In states, interest groups can induce referendums, or the voting on a specific policy rather than a person to vote on said policy for us.