It is a mediatory body which helps to settle disputes between workers and management.
Additional answer
A more likely purpose is that they fight for their members' rights and better pay. To say they are mediatory bodies forgets that often a mediatory body is needed to help settle a dispute between a union (representing the workers) and the management.
an assosication of workers in an industry or groups of industries who join together in their common interests to regulate relations between employers and employees
Central Council of Trade Unions was created in 1921.
Trade Unions Forum was created in 2002-04.
trade unions had bettered the working conditions of the working class
Union of Trade Unions of Chad was created in 1991.
Confederation of Christian Trade Unions was created in 1904.
Zambia Congress of Trade Unions was created in 1965.
Iraqi Federation of Trade Unions was created in 2003.
Confederation of Estonian Trade Unions was created in 1917.
Vanuatu Council of Trade Unions was created in 1985.
Jamaica Confederation of Trade Unions was created in 1994.
National Organization of Trade Unions was created in 1973.
Malawi Congress of Trade Unions was created in 1964.