Let me make an obvious opposing argument: what is the social benefit in allowing people to gamble on the outcome of a sports match, or any other event? We encourage investment in the market in theory to allow companies to grow and produce things many people might benefit from (how well that works is another thing...). Gambling as far as I can see is net negative to everyone but the winners, and not entirely positive to them. Imagine if your next door neighbor dropped a cash-filled envelope that you found - lucky you? And if that was your neighbor's rent money? Like a lot of scams, the 'value' is only accrued by fleecing rubes, and it also creates a new class of super-bookies, which also not positive.
You're asking the wrong question, in a free society (supposedly) people are allowed to do whatever they want by default. They're not perimtted to do things, they're forbidden from doing specific things.
Is gambling a neg-negative? why do you care? how is that relevant? Things shouldn't be forbidden because of net impact, specific harm needs to be outlined and addressed. Most of the time, there are more specific problematic behaviors that should be legislated against, not gambling.
In your example, that person spending their rent money could maybe addressed by the law? Or if someone spends their family's savings on a bet, that specific behavior can be addressed. If you think about it, this lazy approach doesn't address root causes. Maybe that guy's rent money, or family's savings, he could have blown it on a fancy car, no law against that.
Conversely, what if someone bet all their money on a stock option? People kill themselves over this, but it isn't illegal. you see how the entire approach is crooked and lazy? categorizing "gambling" addresses the reactionary emotions of the crowd, it doesn't address root causes, it doesn't evaluate nuanced situations.
It isn't betting or speculating that is the problem.
The social benefit is that it gives a controlled outlet for the need to gamble when managed by the government.
One major utility of prediction markets is to get good estimates of probabilities of future events.
That's an interesting idea, though of course while we've done that with crop or livestock futures, we seem to have coped pretty well without betting on absolutely everything until now.