I know literally nothing about the Lunar Orbital Platform-Gateway or NASA's current plans to return to the moon beyond what I've just read in this post and the linked articles, but my first guess is that the LOP-G's primary purpose is to act as a funding anchor.
NASA's budget and priorities are constantly at risk with every new Congress and every new presidential administration. The Bush administration wanted to return to the moon by 2015, but then the Obama administration defunded that mission after concluding it was over budget and behind schedule. Now the Trump administration wants to go to the moon again, but in a few years that could change once more.
If NASA can get a lunar orbital platform built quickly for the ostensible purpose of serving as a gateway to the moon, then it'll be much harder for politicians to later justify defunding moon-related missions because then they'll have wasted that big beautiful expensive lunar orbital platform.
So as long as the lunar orbital platform exists, it serves as a powerful anchor in funding debates that will help ensure NASA's moon missions continue to be funded, and could even help them get future funding for Mars missions ("We can use the LOP-G as a relay point for a Mars mission, so it's less expensive overall if we go to Mars now than if we wait!").
But that's just my guess. 🤷♂️