Osgood
Hero
What is the importance of cons beyond catering to those who have already bought into the whole gaming experience and, then, only the fraction who will take the time, expense and hassle of participating in a con? I've never understood it, so that piece would need a lot of clarification. Prior to the instant transmission of information that is the internet, I can understand that cons were an excellent venue for the dissemination of information to a large group who could then get that info out to others, but we don't need that anymore.
To my mind, the importance to existing customers is minimal. Sure it was always nice to get early previews of upcoming products and talk with the developers, but you are right there is little importance in a presence at a con. My concern for the marketing has always been the future of game.
The importance is for the huge percentage of attendees who have never played D&D. Odds are good that they have heard of D&D (certainly better odds they've heard of D&D over Pathfinder, Savage Worlds, or what have you), so seeing a booth or the cool play area they had in previous years might encourage them to give it a try. New players are needed for the game to thrive into the future. You hit the nail on the head, many people try out games at a con, but its not easy to try out D&D at Gen Con.
As a side note, not every D&D fan is plugged into online D&D news. In my gaming group I am the only one who bothers to look for D&D news online. For the others, those presentations were the only time they heard about upcoming products. I honestly couldn't tell you why, they seem to spend all kinds of time on social media, but they never bother to look for gaming info.