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.
The Trade Unions were banned 2 May 1933 .
Independent Trade Unions of Croatia was created in 1992.
Council of Nordic Trade Unions was created in 1972.
Centre of Indian Trade Unions was created in 1970.
Swaziland Federation of Trade Unions was created in 1973.
Swiss Federation of Trade Unions was created in 1880.
Irish Congress of Trade Unions was created in 1959.
Confederation of Malta Trade Unions was created in 1959.
Botswana Federation of Trade Unions was created in 1977.
Confederation of Mongolian Trade Unions was created in 1927.