Although the link between interest groups and campaign money is certainly important -- many would say way too important -- interest groups do serve a critical beneficial function.
I worked on a Senate Committee for several years. Congress depends on interest groups to provide a range of advice based on the experience of their members. For example, let's say there's been an increase in street crime, and someone has proposed that all cell phones should have a special 911 button on them. A bill would be introduced and sent to the appropriate Committee, and the Committee would hold hearings. Various interest groups would be invited to testify at the hearings. They would each put together a detailed analysis of the good and bad points of the bill. For example, they might point out that 911 calls on a cell phone are often routed to someplace far away from where the caller is located.
In addition to testimony at formal hearings, interest groups are always happy to answer questions informally.
Few people really understand the power of interest groups. People know they exist, but do not realize how big a role they play. Interest groups don't just lobby for legislation and whine and dine politicians. They actually write legislation, and give it to politicians to sponsor. For example, the legislation popularly known as No Child Left Behind was actually commissioned by and basically written by an interest group called the Business Round Table, then introduced to Congress where it was voted on based on a whiff of information. Few that voted for it or championed it actually read it, it was not a senator or congressperson that wrote it, and yet it became law.
Well interest groups are the ones that fund a lot of the politicians' campaigns, so its usually an exchange of some sorts. The interest groups push for certain legislation to go through Congress, and then Congressman and the President rely on their support in their next election. For the Judicial system, it does not really work the same way, since judges are appointed by the president, and can serve for life. Since the president is the one to appoint certain judges, interest groups can decide to support a certain presidential candidate and fund his campaign if they know what type of judges he/she will appoint when he/she is president.
In addition to campaign donations, interest groups often have full time lobbyists who already have working relationships with the members of Congress, and so these groups tend to have more influence over legislative wording, and in this way can protect and/or benefit the company or industry they are lobbying for. They have purchased access on a continual basis, and are much more able to influence the outcome of legislation than John Q. Public is.
Let me address the influence of special interest groups on the Judicial Branch of government first, as it is quite elementary.
Ideally, there is little to no outside influence on judicial processes. Day-to-day functioning of government involves Executive-Legislative interaction. Once judges (in the federal system, anyway) are nominated by the president and confirmed by the Senate, any further political interference with the workings of the judicial system is largely avoided. That is not to say that special interest groups do not attempt to influence the judiciary, as they clearly do on core issues like capital punishment, abortion, and gun control, but in order for the judicial process to function, it needs to be insulated to the extent possible from political interference.
The influence of special interests on the Legislative Branch of government, on the other hand, is substantial, both by design and in practice. Congress -- and I spent 20 years working as a congressional staffer in both the House of Representatives and the Senate -- was intended by the authors of the United States Constitution to be that branch of government most directly answerable to the public. Certainly, levels of influence on the part of special interest groups can prove excessive, but limiting that influence is politically, legally, and morally difficult. Any individual or group that seeks to influence the legislature is by definition a special interest. That are some are large and well-resourced while others are small and lacking in resources should make no difference, although, in practice, it often does. The point, however, is that lobbying is an inherent and, much of the time, positive element of government.
To learn more about interest groups, check out this video:
Interest groups do not necessarily influence the Judiciary but they very heavily influence Congressmen. A congressman's number one goal in their political career is to get reelected while an interest groups number one goal is to get what they want enacted into law. Therefore, interest groups lobby Congressional Committees in order to get what they want accomplished. In exchange for this Congressional loyalty, interest groups offer Congressmen funding for their campaigned, as well as extremely large voting blocks for the Congressmen.