To quote Lisa Simpson, 'adding a new character is a shameless attempt to boost low ratings', and with that said, Scrappy-Doo is essentially the 'Poochie' of the 'Scooby-Doo' franchise. By the late 70's, the show was becoming tired, and repetitive, so Scrappy was added in hopes that the ratings would improve. While this kept the series going a few years longer, Scrappy pretty much ruined the show's dynamic. Basically, he would instigate the villain they were up against with his usual 'lemme at 'em, I'll splat him' taunts, or encourage Shaggy, and Scooby to 'slat the ghostie' against their will.