In most Soviet Bloc countries, Labor Unions were under the control of the Communist party. If the unions had existed before the country became Communist, they were quickly co-opted or suppressed and replaced with a union that the party controlled. Rather than representing the concerns and grievances of workers to party or government authorities, the labor unions functioned as a way for party ideology, doctrine and propaganda to be passed down to groups of workers. When independent unions like Solidarity in Poland appeared against the wishes of the party, it was a huge change in the nature of Communist party rule, and ultimately lead to it's downfall.

Source: Anne Applebaum's "Iron Curtain"