In those days the heroes of Tamil Cinema used to sing the songs with their own voice. In fact, only persons who can sing in their own voices had the chance of becoming heroes. Thiagaraj Bhagavathar, PU Chinnappa, TR Mahalingam are some of the popular heroes of those days who sang in their own voices and dominated in the field of cinema.
Slowly the trend changed and heroes sang the songs in the films with dubbed voices. Voice dubbing was even used for speech and the languages bar was overcome by dubbing voices to the heroes and heroines.
Nowadays, again there seems to be a change in the trend and Music Directors are trying to make the heroes sing in their own voices.
Singing of the heroes in their own voices will give additional publicity to the films of course at the cost of the chance to the play back singers.