Two particular things come to mind.

1. There's a category of codeshare arrangement that genuinely enables a flight to take place at all, especially on thinner routes or those that present specific technical challenges. It may be that no airline of either country concerned can attract enough traffic on its own to make the flight work, but one airline from each country can do so if they in effect share an aircraft. Permission for such an arrangement may be on terms that still require both airlines to compete against each other despite sharing the same physical aircraft. It may be that each airline can sell space on the codeshare flight within its (and its country's) existing traffic rights, including attracting behind and beyond (including domestic) traffic, without these having to be renegotiated. This could make it easier to work than if only one airline can sell the flight but doesn't already have the rights that would be needed to get enough traffic onto the aircraft.

2. Even for traffic that's entirely within an arrangement like the JBA, there are things like the Fly America Act: AIUI, you can buy a ticket on AA6939 but not on BA178. (Although it may be that this position has now moved on.)