The main benefit would be the ability to offer public Internet-facing services through a NAT, even a carrier-grade NAT that lumps multiple ISP customers behind a single IP address. (You would need some kind of cooperation from the NAT to be able to allow incoming connections on a port range.)

JdeBP's post provides a good answer about the SRV records. The short version is a single DNS packet can contain both A/AAAA and SRV responses.