What's New, Scooby-Doo?
Dr. Ostwald created them as part of a virtual reality mystery he created for Velma. He brought her into his game's booth and briefly demonstrated the game to her and how it worked. He then seemingly turned it off so that Velma could come back and play later, but he had actually sent her to a virtual reality version of the very building she was in. As a result, Velma was the only true member of Mystery Inc. that participated in the mystery, since the rest were all virtual reality projections.
The virtual Mystery Inc. accompanied Velma as she attempted to solve the mystery of the Osomons, small furry monsters from a game of Ostwald's that supposedly turned hostile and started running amok. Though the virtual gang behaved similarly to their real counterparts, they had glaring holes in their characterization that ultimately tipped Velma off. First, Fred wanted to stick together and search for clues while the real Fred always wants to split up. Second, Daphne had no issue wearing shoes that didn't match, something the real Daphne wouldn't do. Third, Shaggy and Scooby actively volunteered to be live bait; the real Shaggy and Scooby would never do this and always need to be bribed with Scooby Snacks. The final clue was when one of the Osomons bit the virtual Shaggy's arm. He screamed "Toinks!", when the actual Shaggy would've said "Zoinks!"