A recent article explained how political parties are organized in the United States. All individuals registered by political party have a say in the operations, policies and decisions made by the party they have chosen to become a part of. Unaffiliated or independent voters do not have this right.
Political parties tend to be heavily criticized because those who are knowledgeable of how the system is supposed to work frequently hi-jack it and go in directions the grass roots members oppose. Most party members choose not to participate because of the bad behavior of some of the leaders. This allows the bad boys to become even more powerful.