The project-labor agreements themselves are one of the reasons why construction labor, in particular, is among the most powerful of unions, according to Gary N. Chaison, a professor of industrial relations at Clark University in Worcester, Mass. They’re strong because they can control who enters into the profession, he said. Unions in construction control work. They control who gets jobs. By controlling the jobs, they control the workforce. By controlling the workforce, they gain power.
Source: In R.I., unions still rule