Why do Catholics have Masses said for the dead?

Masses for the dead began with the belief that if these souls were in Purgatory, then Masses would help in the purification of their souls. God in his divine mercy and love would then bring these souls into Heaven. "This sacred council accepts loyally the venerable faith of our ancestors in the living communion which exists between us and our brothers who are in the glory of Heaven or who are yet being purified after their death ..." (Dogmatic Constitution on the Church, no. 51). What greater way to be in communion with our deceased brothers and sisters than while we are thinking of them and praying for them during the Eucharist?

http://www.catholicherald.com/saunders/03ws/ws031030.htm