Apple sells "carrier-specific" versions of their phones, as well as "sim-free" versions.
If paying in full, these are the same price (e.g. a Verizon or AT&T 64GB iPhone SE costs $499, as does a sim-free 64GB iPhone SE).
I understand that the sim-free iPhone does not work with Sprint, so if I wanted to use it on Sprint, I would have to buy the Sprint-specific version.
But in the case of Verizon, T-Mobile, and AT&T, why would anyone buy the carrier-specific version instead of the sim-free version? Do they offer any advantage? Why does Apple even sell them?