I agree that playing teams outside the normal league is a compelling reason for those competitions, but I think the biggest potential benefits aren't tied to *who* you play, it's mixing up competitive stakes and motivations. One of the things that plagues American sports is that by having a singular point of success at the end it creates sometimes perverse incentives when combined with the other rules and realities of the league. Tanking in MLB(though improved over a decade ago) and load management in the NBA are prime examples.
One of the ways you can address that is to mix up competitive stakes, tie that end goal to other competitions, or provide some other type of reward to give the competition meaning. This first edition for the NBA doesn't do a lot of that, though I understand their desire to trojan horse it a bit by making some games dual-purpose and draw corollaries to divisional standings in terms of adding meaning. But off the top of my head, what if the Eastern and Western conference teams played separate march madness-style brackets at a neutral site for a guaranteed playoff spot? What if those winners played for a cash prize and a lottery pick? Or to take a different tact, what if you had a chunk of games(15-25?) that were designated as part of a different competition, Champions League style, with the best record(s) getting a playoff spot the next year(or a bye if they made it on merit) and the worst record(s) being ineligible for next year's playoffs regardless of record?
The point is not the specific nuances of those ideas(though without dwelling on it that first one sounds pretty fun), but that once you recognize the idea that all the competition specifics we currently have are a combination of arbitrary and optimized for entertainment, that you can make something that's even better and fixes some current problems by embracing more complexity and separation in those competitions.