What is the primary purpose of special-interest groups

The purpose is to communicate, debate or try to convince parliamentarians or government executives to make a particular decision to address particular or general interests.

Explanation:

Interest groups is the name given to the activity of influence of an organized group with the objective of directly interfering in the decisions of the public power, especially of the legislative power, in favor of causes or objectives defended by the group through an intermediary

A Special Interest Group (SIG) - a term which was first introduced in 1961 by the Association for Computing Machinery - is a community that belongs to a bigger organized body. Its primary purpose is to progress in a particular section of knowledge, education or technology where members collaborate to create solutions inside their specific field. Also, they might produce reports, meet, and arrange conferences.