This question is specific to the Apple/Google Contact Tracing framework. See https://www.blog.google/documents/56/Contact_Tracing_-_Cryptography_Specification.pdf for the cryptography spec.
In this spec, note the Rolling Proximity Identifier (RPI) is generated from the Daily Tracing Key and the TIN using HMAC.
My understanding is that the function to generate RPI only need to be a one-way function with deterministic output from a given input. Then why in this spec a HMAC is used instead of say a SHA256 function with N iteration?