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).
It depends on how much other people care about your data, and how much physical control you have over the devices. If you’re in “nation states would like to have it” territory, you should never have unencrypted data at rest, or on the wire. If you’re a regular home user and all your computer stuff is inside your own house, you’re probably fine. In between, there’s a lot of possibilities. Encryption is cheap.