How do characters having almost no impact on the plot of the movie improve the movie? I mean, why?
There is more to a movie than plot. Movies are not created for the sole sake of a Wikipedia plot summary.
I agree
This is to say nothing about the fact that they do impact the plot.
I agree if you’re talking about Poe, but not about Finn. Still, both of them got a lot of good character development which is a lot more important.