Why do people with similar views on an issue or cause form special-interest groups?

If they surround themselves with other who believe the same as them, it reassures their beliefs. An example of this is the Anti-Vax people. Instead of surrounding themselves with other who can argue their views, they are around those others who believe in the cause.

Answer: If they have similar views on a issue or cause then they could work together and find other people with the same view that they have to better the issue/cause.

Explanation: These interest groups represent interests that support and lobby for areas of special need. For example, the Sierra Club focuses on protecting the environment as well as the wild places on earth. They also focus on the education of people on preservation of the environment.