What is the advantage to buying a carrier-specific iPhone over a sim-free version? 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&amp;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&amp;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?
Apple sells carrier-specific iPhone models so that the carrier can do subsidizing or financing. The carrier-specific ones comes with a pre-activated SIM for the carrier that you choose. There is no advantage other than convenience when paying in full (other than the Sprint requirement). One thing, though, is that the SIM-free version doesn't come with a SIM, so you need to get one from you carrier, which might cost money. The SIM-free and T-Mobile versions come factory unlocked, and the others might not.