There are many different kinds of advocacy organizations. Some groups advocate for broad causes like the Sierra Club does for the environment or NARAL/Pro-Choice America does for women’s health care. Some represent a specific group of individuals, like labor unions who represent their workers, or veterans’ groups who represent their members.
No matter what these groups do or represent, the usually have members. And they enlist these members in the advancement of their collective cause. They do this by pooling their financial resources and giving of their time and energy.
Some groups work only in electoral politics, while others work only in the public policy arena. Some lobby elected officials, some don’t. No matter what kind of agenda advocacy groups pursue, they are running a campaign (be it electoral, legislative, public relations, etc.)
Advocacy groups don’t get involved in politics because they like the politicians they endorse; they get involved as part of a broader strategy to advance a policy agenda. Thus it also helps for you to believe in the cause of any advocacy organization for whom you choose to work.
Getting started in advocacy is typically much easier after working on an electoral campaign or with legislative experience from the Hill. While some opportunities do exist for those right out of college or changing careers, many organizations and firms are looking for individuals who have developed specific skills that you can acquire on the Hill or or campaigns.
Working in advocacy is often a great opportunity to engage in a full range of campaign techniques and develop transferable skills that can be used anywhere in politics. Thus, it's a great way to broaden and deepen your resume when looking to advance your career in professional politics.