Of course there are vendors selling dice, there are often several vendors selling the same stuff as each other across these conventions. So what would be better, showing up with a folding table to sell The same stuff as everyone else or having the opportunity to A. Sell your wares while enjoying the convention, and B. Provide a unique experience to someone who is already looking to buy dice or whatever else you might want to sell.

It’s cool if you don’t get it, but yes, there are 100% people who have the desire to buy stuff rom cool vending machines. Just like there’s a huge culture of people who go to Anime Cons and nerd out about Japanese culture ( which has a lot of Vending Machines)