Why are lobby groups or interest groups so important?

Lobbying groups, or interest groups, usually operate in Washington, DC, and through proximity to representatives and substantial campaign contributions, they can impact political policy. For, the aim of interest groups is to raise money for politicians whom they believe will further their particular interest.
Lobbying groups have a variety of interests—some are corporate, while others are social; some are international, while others are local. The National Rifle Association (NRA) is a lobby for gun manufacturers. EMILY's List is a lobby that promotes electing more women to public office. The American Israel Public Affairs Committee (AIPAC) is a lobby dedicated to strengthening and protecting the relationship between the United States and Israel. These special interest groups tend to work more directly with one party than with another. For example, the NRA is favorable among Republicans, while EMILY's list is more popular with Democrats, and with Democratic women in particular.
Lobbying groups are usually identified as Political Action Committees, or PACs. More recently, we have seen the rise of the SuperPAC, which is technically known as an independent expenditures-only committee. SuperPACs raise unlimited sums of money from substantial groups, such as corporations, unions, influential associations, and wealthy individuals, then pool that money to advocate for or against a political candidate. Very often, SuperPACs use their funding to buy airtime on television. SuperPACs, like PACs, can be either liberal or conservative in their political ideology and aims.
Years ago, senators John McCain (Arizona) and Russ Feingold (Wisconsin) introduced the McCain–Feingold Act, which sought to address the unbalanced influence of these groups in politics. McCain is a Republican and Feingold, who is no longer a senator, is a Democrat. Thus, this was a bipartisan bill. However, it still failed to gain support, continuing to allow lobbying groups more influence, through campaign dollars, over elections.
After the Citizens United vs. Federal Election Commission (2010) decision in the Supreme Court, an even wider door opened for unlimited campaign dollars, due to the court's classification of campaign contributions as a form of protected speech under the First Amendment.