When your opponents are Panama (5th or 6th best in Concacaf), Martinique, and Nicaragua you shouldn't worry about winning. The squad underperformed. This is when you break in new players into formal competitions. As the GC immediately before the WC, there is no other reason other than finding players to fill the bottom of the roster and get your WC squad playing together.
Arena is doing both. The guys he is bringing in shouldn't have made a difference against such weak competition. If the US makes the final it plays CR and Mexico, two of the final 16 in Brazil. This will be the biggest tests short of one or two friendlies that may happen against 2018 prep squads from the big powers, like Germany or Argentina.
Arena can't get his best 11 together because many are in Europe trying to win time at their clubs which will be more important than playing in the GC.
Evaluations also happen in training. Guys like Miazga are on the radar for bench positions or roster spots due to injury or loss of form from guys ahead of them. Right now Miazga isn't making the roster, but he has a year to make the case an this is his opportunity.
He is shrinking the evaluation pool, but at the same time giving some guys time to get back to their clubs. The GC was set up as two tournaments, a B/C level group stage and an A/B level knockout round. I have no issues with how Arena is playing this.
