A PTR record answers the reverse question a normal DNS lookup doesn't: given an IP address, what hostname does it resolve to? Many receiving mail servers check that a connecting IP has a PTR record at all, and that it resolves to a hostname whose own forward A record points back to the same IP (forward-confirmed reverse DNS).
A missing or mismatched PTR record is one of the most basic deliverability checks a receiver performs, often before any content or authentication evaluation happens at all. It's controlled by whoever owns the IP address block, not by DNS on the sending domain.
For a dedicated sending IP, the PTR hostname is typically set to something clearly associated with the sending infrastructure (e.g. a subdomain under the provider's own domain), configured once at IP provisioning time.