What's in it for us, supporters? Fuck all, really.
What's in it for the owners? Billions of pounds.
To understand the attraction, you have to understand that billionaires finances are wholly different than ours. They don't collect paycheques/dividends. This is why few stocks these days pay dividends - dividends are coveted by average investors, but useless to billionaires they don't want income. They live on credit extended to them based on the value of their holdings. So when Spurs' values doubles, Lewis' values doubles and his access to capital greatly increases. That capital can be accessed to fund further investments. Money begets money.
What's in it for the club? A more competitive landscape. A closed structure Super League would enact salary controls - the billionaires don't want to frivolously waste money needlessly on players. They'd still pay the highest salaries in the world (like the NBA) so would get the best players - but collectively bargaining salary controls provides a fixed cost of labor that can be pegged to the fixed value of collectively bargained revenue (broadcasting, merchandising, etc.) And guarantees profits and growrh for owners.
In a fixed salary structure league, who "succeeds" and "fails" is about who is the best and most efficient at squad building. So, with out current setup, we'd probably not thrive, obviously. Liverpool, probably would. But the chances of us winning would be much higher in that league than in our current league, where we have to be exponentially more efficient because we have to overcome a base spending inequity. In a fixed salary Super League, we'd just have to be better at spending the same amount of money we're allowed to spend.
This is why the Knicks and Cowboys are not successful, despite being the vastly most valuable teams in their sports - they're shit at building teams and cannot leverage their value advantage to offset their lack of squad building talent.