Without SSL on the LAN side of a reverse proxy, I presume that all traffic between the server and the reverse proxy is unencrypted and, thus, accessible to any device on the LAN.
Which specific scenarios result in this being a concern? The primary concern that I can come up with is if you know that there are untrustworthy entities connected to the LAN (untrustworthy devices, or perhaps malicious individuals).
Security comes in layers. If someone compromises your DNS server, or switch, (or does arp poisoning, etc etc) for example, but not the reverse proxy, (and it resolves backend via DNS and it doesn’t validate/pin certs), they could intercept the traffic transparently. If you have SSL on that link, it massively reduces the attack potential.