The biggest stars of a TV show are the good guy characters, the ones we are supposed to relate to and root for. At least, that was the tradition in TV for decades. But it’s not nearly as much fun to play the good guy as it is to play the villain. That’s where the evil twin comes in. The much-used plot device opened the way for mysterious dramas that could only be explained by an evil twin. It gave a “good” character room to do things he or she otherwise wouldn’t do, and it baffled audiences until the big reveal. And there was another benefit: it was a way to have two characters on for the price of one actor. For soap operas, these plots could go one for months or even longer.
In 1983, Brian Patrick Clarke began his run as nice married guy Grant Andrews and his Russian spy doppelgänger Grant Putnam on General Hospital. Ellen Wheeler picked up the dual roles of Marley and Vicky on Another World in 1984, eventually leaving the part to Anne Heche. David Canary would also start his decades-spanning work as ruthless businessman Adam Chandler and his gentle, artistic brother Stuart on All My Children that year. But perhaps most interesting was the case of Frannie and Sabrina Hughes; the half-sisters appeared on As the World Turns between 1985 and 1987, and the dual role was originated by a not-yet-famous Julianne Moore.
And that’s only a few years. Evil twins have been spicing up soap operas for decades, which you can read about at mental_floss.