Congressional committees act as legislative gatekeepers -- virtually all bills must pass through one or more committees before reaching the floor of the House or Senate.   Interest groups therefore strive to procure favorable consideration of bills they like, favorable amendments to bills they find partially objectionable, and rejection of bills they oppose.  In exchange for this, legislators receive campaign contributions and sometimes investments in their district or state.  Executive agencies receive funding from and are overseen by Congressional committees.  In exchange for cooperation with a commitee and its interest group allies, they receive political protection in budget battles and the like.  In the case of the contractor--supplier relationship between an executive agency and an interest group, the alliance between them works to protect the economic interest of the supplier in continued contracts and the institutional interest of the agency in continued funding.