America’s real-life kings of professional sports — its leagues, teams and media empires — are betting big on the make-believe of daily fantasy sports, which throw the traditional fantasy season into hyperdrive, add glitzy prizes and sidestep the country’s online-betting bans.

Daily fantasy site DraftKings said this week it had received a $300 million investment from Fox Sports, Madison Square Garden and the national leagues of baseball, hockey and soccer. Its chief rival, FanDuel, pocketed $275 million this month from NBC Sports, Time Warner’s Turner Sports and other investors (including Google and Comcast).

It’s the biggest outpouring yet for the young prize-dangling industry, which started as a sports nerd’s online distraction and remains a gambler’s niche.

But for some of the sports world’s biggest players, the business hides a surprising upside. The online games don’t just make gobs of money — they can help hook sports junkies, and the next generation of fans, into paying up for the real thing.

“The more people play,” Fox Sports president Eric Shanks told Re/code, “the more they consume our product on TV.”

Fantasy sports, particularly fantasy football, have long been a part of the American mainstream, with its workplace leagues, online drafts and fantasy-roster specials on ESPN. More than 56 million people in North America will play fantasy sports this year, up from 12 million in 2005, Fantasy Sports Trade Association data show.

The traditional fantasy match demands a season-long commitment, and players often bow out early if their chances go bust. But with the daily model, a player can draft a team in the morning, stay glued to his or her phone in the afternoon and suffer the result (or pocket the reward) by day’s end, in time for the next match. If season-long fantasy is a long-term investment, daily fantasy is a slot machine.

On Monday, DraftKings’ baseball bets ranged from a 60-player, high-stakes brawl — entry cost: $5,300; payout: $292,500 — to a 25-cent game promising $1,000 to the winner of its 2,500-player battle royale. The site also stages heavily hyped tournaments, complete with brackets and championship rounds: This week in the fantasy baseball Kings Cup, “CSURAM88” will take on “dinkpiece” for the $50,000 top prize.

The fantasy contests — in which a player’s made-up team is scored on how well its athletes played in actual matches — are easy to play, hard to master and heavily dependent on luck. But astute players can get ahead by knowing the day’s match-ups, and the sports world knows that means added devotion to their moneymakers and corporate brands.

FanDuel says its more than a million paying players watch and read 40 percent more sports content after joining. Those fans may also be more likely to fork over cash for love of the game: Daily fantasy players will spend an estimated $257 this year, compared with $162 for traditional fantasy leagues, Fantasy Sports Trade Association data show.

Founded in Scotland in 2009, FanDuel says it now offers more than 12,000 open leagues — covering football, baseball, even golf and mixed martial arts — and paid out more than $500 million in prizes last year. Like its rival DraftKings, which launched in Boston in 2009, newbies are spotted some free betting cash, then the sites take about a 10 percent cut on players’ future bets.

FanDuel and DraftKings both expect more than $1 billion in payouts this year, which would net them about $100 million each in commission revenue.

How has this digital empire survived in a country that explicitly forbids online sports betting? Daily fantasy matches are classified as games of skill, not chance, and are thus exempted from the Unlawful Internet Gaming Enforcement Act, the 2006 law passed before the daily model was born.

By speeding up play, sites such as FanDuel were able to offer bettors a more tantalizing risk-reward payout, especially for those seeking the instant gratification of now-banned online-poker and sports-gambling sites.

The daily fantasy model helped the start-ups escape the pull of massive media engines at CBS, ESPN and Yahoo, which dominate season-long fantasy leagues. DraftKings’ chief executive, Jason Robins, said in March he expects his company could one day be worth more than $10 billion.

The sites have quickly become major forces in sports advertising. FanDuel, one of ESPN’s biggest buyers of TV spots, had its logo on the shorts of Floyd Mayweather Jr. when the boxer beat Manny Pacquiao in May. DraftKings’ logo has been emblazoned behind home plate at several baseball stadiums and was on American Pharoah last month when the racehorse won the Triple Crown.

That helps explain why investing in the daily fantasy business can seem like a win-win for media conglomerates. Fox Sports is investing $150 million into DraftKings’ latest haul, but some of that cash may make a round trip, people close to the deal told Re/code, because the company agreed to buy $250 million in ads on the TV network over the next three years.

Either way, America’s biggest sports leagues see lots to love in a new wave of fantasy-emboldened sports fanatics. Last year, the National Basketball Association signed a four-year deal with FanDuel and took a minor stake in the company. Major League Baseball, the National Hockey League and Major League Soccer boosted or made new investments in DraftKings’ newest funding round.

The sports and media world sees daily fantasy sports as a quick in for a target market: Young, tech-happy sports fans with the cash and attention spans to support modern sports. They’re often used to staring at their phones or computers while watching live games, anyway.

As Nigel Eccles, FanDuel’s founder and chief executive, told Forbes, “If you take something people love and make it faster, it always works.”