Sales staff can help compare T-Mobile with AT&T or Verizon (though best buy no longer supports T-Mobile activations, check current store availability).
If you are a legacy Sprint customer transitioning to T-Mobile, or if you need to resolve billing issues, a corporate T-Mobile store is usually more capable. sprint store vs best buy
Best Buy allows you to compare different phone brands (Samsung, Apple, Pixel) and carriers, making it better for shoppers who aren't loyal to one brand. Sales staff can help compare T-Mobile with AT&T