The United States Men's National Team and Major League Soccer are permanently joined at the hip. The two entities have long been joined literally through Soccer United Marketing (SUM) and shared business interests, but even after that partnership ended, the two groups do not exist independent of each other.
The USMNT is the face of men's soccer in the United States and MLS is the league trying to do the same. Almost all of the best players on the national team now and in it's post-1994 history have spent at least some time in MLS.
When the USMNT missed out on the World Cup in 2018, a laughable failure that brought the entire system into question, that had an undeniable effect on MLS. The profile of soccer, and particularly American men's soccer, that comes with a USMNT World Cup run is undeniable.
In 2014, the nation rallied around the men's national team, with huge watch parties and support for the team. It turned the likes of Clint Dempsey and Michael Bradley, and at least for that month, turned them into national superstars.
Those stars just so happened to be developed in and returning to Major League Soccer.
It's a special type of bump for the league that only a World Cup can provide. That was lost in the 2018 edition. The tournament was still a huge deal, even in the states, but it didn't have the same weight in the general American sports consciousness.
For Major League Soccer to propel it's growth, all in the lead-up to an exponentially bigger World Cup in 2026, the USMNT's qualification for the 2022 edition is massive. That exposure is huge for fan support domestically, and promoting the league's best, including Walker Zimmerman, Miles Robinson and Jordan Morris, on the biggest stage in sports.
The symbiotic relationship between the USMNT and MLS makes every World Cup cycle huge, not only for USMNT fans, but fans for every MLS club (albeit there's already a ton of overlap in that space). If the US can finish the job this week, potentially sealing the deal in Orlando at Exploria Stadium against Panama, it will provide a potential boon for MLS in a growth period for the league.